Why It Works
- Caramelizing the onions slowly in butter until they're rich golden brown (but not so dark as to taste bitter) produces the sweetest, most flavorful results.
- The homemade or store-bought chicken stock used here is typically better-quality than the store-bought beef stock many recipes call for.
- Asian fish sauce, cider vinegar, and sherry add depth and complexity to the broth.
In my very first test for this French onion soup recipe, I had a realization: There's no good reason why so much bad French onion soup exists in the world.
The thought arrived after I had caramelized a variety of onions in butter, then added some homemade chicken stock and let it simmer for a while. Aside from salt, I had put nothing else in the pot. And yet, despite being such a bare-bones version, the soup tasted like one of the best French onion soups I'd ever had.
It made me reflect on all the crappy French onion soup I've been served in my life. Bowls upon bowls of thin burnt-onion tea, the sharp flavor of caramelization gone wrong—something no amount of melted cheese can hope to correct. With nothing more than onions, stock, and salt, it's possible to make one of the most delicious broths in the world. So why are good versions so rare?
The answer lies right there in the question: In the case of such a simple soup, its success or failure comes down to the onions and the broth. Do them right, and you have a masterpiece on your hands. Do them wrong, and it's all lost. Sure, there are things we can do to elevate the soup even further, but they can't stand in for a good base. And the croutons and melted cheese—a requirement for soupe à l'oignon gratinée, and what most Americans think of when we think of French onion soup—should be a bonus, not a crutch.
Choosing and Caramelizing the Onions
One of the most common beliefs surrounding French onion soup is that the onions must be cooked to a deep, deep, dark, dark mahogany brown. I'm going to start off by calling BS on that premise. It's not that I think it's wrong to caramelize the onions darkly; I just don't think it's necessary for great results. I also think there's a big risk in going very dark: Unless you're exceedingly careful, it's very easy to introduce unpleasantly bitter flavors to the onion—one of the culprits in so much of the bad French onion soup out there.
In test after test, I found that great French onion soup can be made with more lightly caramelized onions. The deep, sweet flavor that we want arrives long before they turn the color of dark chocolate. And, as I researched other French onion soup recipes, I discovered I wasn't alone in this realization. In fact, some of the people I trust most on this topic have said exactly the same thing.
Here's the acclaimed chef André Soltner in the preface to the onion soup recipe in his Lutèce Cookbook:
"Do not think for a minute that this is the notorious soup you get in Paris workingmen's cafes at five o'clock in the morning.... In those soups the onions are sautéed until they are black, and the soup is dark and bitter. Some people like it that way—which I can never understand."
He then proceeds to instruct us to cook the onions until golden brown. Julia Child, in Mastering the Art of French Cooking, meanwhile, says to cook the onions until they're an "even, deep, golden brown." As far as I can tell, an even, deep, golden brown is not the same as a deep, dark brown. And in The Paris Cookbook, Patricia Wells shares the recipe from Paris's Brasserie Balzar, and makes a point of not cooking the onions nearly as dark as so many other recipes insist upon. Interestingly, her recipe aims only for a pale golden color, even lighter than what I think is the sweet spot.
While Soltner's quote makes it clear that many versions of this soup are indeed made with intensely dark onions, part of me wonders whether some of the trouble has been a result of poorly chosen words that have then become gospel. Take this recipe and this video of Jacques Pépin as examples. In the recipe, he says to cook the onions until "dark brown," but then gives a cooking time of only 15 minutes—an impossibly short time for that level of browning. And in the video, he tells Julia Child to cook the onions until dark, but one look in the pot and I think we can all agree that they're much closer to a rich golden brown, still a ways away from reaching the deeply dark stage many recipe writers call for.
"So what do we really want from the onions? We want them incredibly soft, with a deep, sweet flavor and a color that's a rich golden brown."
So what do we really want from the onions? We want them incredibly soft, with a deep, sweet flavor and a color that's a rich golden brown. Some folks may not agree with this—sometimes the way we've grown accustomed to a food becomes the only way we can imagine it. That's fine: You're free to cook the onions even more if that's what you prefer. What I want to stress, though, is that you don't have to, and you may well discover, as I have, that you find the soup even more delicious when you don't cook the onions to a deep, dark brown.
This still leaves open the question of the caramelization itself. But before I get to that, let's talk onion types.
Choosing Onions for French Onion Soup
One of the first things I wanted to figure out was whether the choice of onion itself mattered much for French onion soup. To find out, I caramelized four different varieties to a rich golden brown: sweet onions (such as Vidalia), red onions, yellow onions, and shallots.*
*Okay, technically not an onion, but a close enough relative to make it worth trying out.
Here are my tasting notes on the caramelized onions:
- Sweet onion: mellow and sweet, with a brightness right at the end.
- Red onion: deeper flavor, with a slightly bitter edge and less sweetness.
- Yellow onion: lots of bright flavor, very mild bitterness, and a sweetness backing it up.
- Shallot: really good balance of sweetness, with both bright flavors and deep, rich ones, and just a hint of bitterness.
It's possible that different samples of each of these onion types might have produced different results, but what's key is that the variation exists in the first place: Some onions will be more bitter, some more sweet, some brighter, some deeper and more complex. Much like with my fresh tomato sauce recipe, mixing them together produced the most well-rounded and complex flavor of all.
The bottom line is that none of the onions were bad, and you can make great soup from any one of them.** And yet I can't help but think, why not get more bang for your buck by using a blend of varieties? I won't go so far as to say that using multiple onion varieties is a requirement of great French onion soup, but I do think that if you're shopping around and have options, there's no harm in mixing it up.
** Red onions take on a bit of a dull gray color compared with the other types when browned, but this doesn't bother me much once they're simmered in the broth.
If not, inexpensive yellow onions are probably your best bet.
Caramelization can be a wildly inconsistent process. Different pots and pans, batch sizes, and fats all change how (and how quickly) onions caramelize.
Let's start, though, with one immutable fact: Contrary to almost every recipe I've read, onions do not caramelize in 15 minutes. They also don't caramelize in 25 minutes. And, unless you're cooking a very small batch, you'll be lucky if they're done in 45 minutes.
Yes, over high heat, onions can brown on the surface very quickly, especially in a pan that's not crowded. But the goal of caramelizing onions isn't just to brown the surface; it's to transform the onions so that they're soft and sweet to the core. Try to rush it and all you'll end up doing is burning them. Here's the skinny: Caramelizing onions, even to the lesser degree I'm advocating here, takes at least an hour, sometimes two. In the oven it took me about three hours at 375°F (190°C), though the oven requires less frequent stirring. (It's also more likely to produce over-browned bits as the onions reduce and their residue scorches on the side of the pot, which makes it a method I'm less enthusiastic about, though not completely opposed to.)
What about all those other variables? In my tests, I found that butter not only produces much more delicious caramelized onions than oil, but also kicks off the browning reactions more quickly, thanks to the propensity for the milk solids in butter to brown.
As for cooking vessels, I had the best results in cast iron and carbon steel, with stainless steel coming in a close second. Enameled cast iron, on the other hand, didn't perform as well. For some reason, the onions are much slower to brown in an enameled pot.
In the enameled pot, the enamel surface turned more and more brown while the onions remained white; I had to keep adding water to scrape up the browned bits before they burned, while waiting for the onions to transform. Adding water isn't inherently problematic—sometimes you have to deglaze, especially in stainless steel, to avoid burning the fond during the caramelization process—but enamel made it necessary to deglaze constantly. Of all the pot materials, cast iron and black steel required the least deglazing, possibly because their seasoned finishes provide a nonstick surface that keeps the sugars off the pan and on the onions.
One technique I wanted to test was using baking soda to speed up browning reactions, which works by raising the pH—foods brown more quickly in alkaline environments.
But even at minimal levels of a quarter teaspoon of baking soda in one and a half pounds of onion, it turned them an unappealing green-yellow color, reduced them to mush, and gave them an unpleasant flavor. I don't think it's worth it.
Sugar is another ingredient that is commonly added to onions to help speed up their caramelization. I did side-by-side batches, and, while sugar definitely helps, I found that even in small amounts, it boosts the onions' sweetness to candy levels. Some people like this, but then again, some people have much sweeter teeth than I do. In most cases, I find that onions have more than enough of their own sugars to caramelize and sweeten deeply and in a balanced way; additional sugar risks taking them over the top. As with tomato sauce, I'd rather use sugar only when it's necessary to correct a particular batch of onions that isn't sweet enough, rather than think of it as a required ingredient.
Where does that leave us? Actually, at a very classic place. To my taste, the most delicious caramelized onions are cooked slowly in butter the old-fashioned way. It takes time and is free of cool tricks, but I think it delivers the best results. This is something Kenji has written about before when describing his shortcut method of caramelizing onions: While there are ways to speed the process up, for a dish as reliant on great caramelized onions as French onion soup, the best method is taking your time to do it right.
The Broth for French Onion Soup
With the onions caramelized, the next step is to add the liquid and simmer it all together. I start with some sherry, since I love that nutty, oxidized flavor with the caramelized onions. You could also use vermouth or white wine, and some folks even use red wine or port. Take your pick; they're all good.
Next comes the stock, which is the second most important component of the soup. Traditionally, the stock of choice for French onion soup is beef stock, but beef stock is very time-consuming to make at home, and store-bought versions are so terrible, they're not worth considering.
Chicken stock is therefore your best option, and it produces absolutely wonderful results. By far the best you can do is homemade chicken stock. Use good caramelized onions and homemade stock, and you'll have a soup that won't even need the cheese to be delicious.
If you don't have homemade stock on hand, store-bought can work well, though it's worth paying attention to the brand. I used multiple types in my testing, and the results ranged from great to horrendous. Take a look at our chicken stock taste test for some recommended brands.
A few sprigs of thyme and a bay leaf elevate the soup even more, but I take it a step further, adding a splash of fish sauce for complexity and depth—don't worry, it won't taste fishy—and a hit of cider vinegar to balance some of that oniony sweetness.
Finishing the French Onion Soup
With our building blocks in place, the final step is fixing up the bowls for serving.
I start by buttering warm croutons and rubbing them with garlic.
Then I spoon a little of the soup into each (oven-safe) bowl. I set a crouton on top and top that with cheese.
Then I ladle in the bulk of the soup, nearly filling up each bowl. I add another crouton on top of that.
And finish it with a very generous grating of Gruyère cheese. I broil the soups until the cheese is melted and browned in spots, then top each one off with a garnish of minced chives, for a little hit of fresh onion flavor to play off the deep sweetness of the cooked onions.
Each spoonful should be a mix of melted cheese, broth, tender onions, and bread. Aside from the time it takes, it couldn't be easier, or more satisfying.
6 tablespoons (90g) unsalted butter, plus more for bread
3 pounds (1.4kg) yellow or mixed onions, sliced 1/8 inch thick (see note)
1/2 tsp kosher salt (or to taste), divided
1/2 tsp, ground black pepper (or to taste), divided
1/2 cup (120ml) dry sherry (such as Amontillado)
2 quarts (1.8L) homemade or store-bought low-sodium chicken stock
2 sprigs thyme
1 whole bay leaf
1 teaspoon (5ml) Asian fish sauce (optional)
1 teaspoon (5ml) cider vinegar
8 bowl-size slices rustic bread, toasted until crisp
1 medium clove garlic
1 pound (450g) Gruyère cheese, grated
2 tablespoons freshly minced chives, for garnish
In a large stainless steel saucepan, or in 2 large stainless steel or cast iron skillets, melt butter over medium-high heat until foaming. Add onions and cook, stirring occasionally, until softened, about 8 minutes. Lower heat to medium-low and cook, stirring frequently, until onions are very sweet and a rich golden-brown color, 1 to 2 hours. If browned onion juices on bottom of pot threaten to burn, add 1 tablespoon (15ml) water, scrape up browned bits, and continue cooking. Season with salt and pepper.
Add sherry and bring to a simmer, scraping up any browned bits. (If using 2 skillets, divide sherry between them, then scrape onions and liquid from both pans into a pot or Dutch oven to continue.) Cook until alcohol smell is mostly gone, about 3 minutes. Add stock, thyme, and bay leaf; raise heat to medium-high; and bring to a simmer. Lower heat and simmer for 20 minutes.
Add fish sauce, if using, and cider vinegar. Season with salt and pepper. Discard thyme sprigs and bay leaf.
Preheat broiler and adjust oven rack to top position. Butter toasts and rub with garlic clove until fragrant. Spoon a small amount of broth into the bottoms of 4 ovenproof serving bowls, then top with half the toasts. Sprinkle some grated cheese on top of toasts, then spoon more soup and onions on top, nearly filling bowls. Set remaining 4 toasts in bowls, pushing to nearly submerge them. Top with remaining grated cheese and set bowls on a rimmed baking sheet. Broil until cheese is melted and browned in spots. Garnish with chives and serve.
Yellow onions are your best all-purpose bet for this soup, but a mix of yellow, sweet (e.g., Vidalia), and red onions, as well as shallots, produces an even more complex flavor.