There's a universal law for caramelized onions: The longer they take to caramelize, the tastier they'll be.
Like all universal laws, this one is meant to be broken.
In the past, I've offered a couple of cheaty ways to caramelize onions without the hours of painstaking stirring involved with the traditional method. Using a combination of sugar (to aid in caramelization), baking soda (to increase the effects of the Maillard reaction), and high heat, with plenty of deglazing, can deliver onions that go from raw to caramelized and sweet in just about 15 minutes (and we've got a video to prove it!).
But here's the sad truth: The methods are faster, but the results are simply not as good as traditionally slow-cooked caramelized onions. They aren't quite as sweet, they aren't quite as complex, and they aren't quite as meltingly tender. No, in order to break the universal law of caramelized onions, we have to call in a pinch hitter. Something that can subvert the standard order of the kitchen universe: the pressure cooker.
Let's back up for a second and take a look at our goals when we're caramelizing onions. There are a couple of things going on. First, there's caramelization: the breakdown of complex sugars into smaller base units that then get recombined into a series of more complex molecules, which simultaneously add sweetness and complexity. Second, there's Maillard browning: the similar reactions that take place between proteins and sugar, adding a rich brown color and flavor to your food.
"Only after sufficient moisture has evaporated can the temperature in the pot rise high enough to start browning and caramelizing in earnest."
Both of these reactions require relatively high temperatures in order to take place. When you're sautéing, the temperature in the pan is largely regulated by the water content remaining in your vegetables. At first, as onions start to expel moisture from inside their cells, the energy in the pan mostly goes toward evaporating that liquid. The pan has a tough time rising above the 212°F boiling point of water. In this temperature range, not much browning or caramelization can take place. Only after sufficient moisture has evaporated can the temperature in the pot rise high enough to start browning and caramelizing in earnest.
But there's a problem: Without the built-in regulation of moisture, it's very easy for the temperature inside the pot to rise too rapidly. Your onions can go from caramelized to burnt in a split second, and even a few bits of burnt-onion juice will ruin the flavor of the entire pot. That's why caramelization is such a slow, painstaking process. The gentler the heat you use, the fewer burnt compounds you produce, and the better the flavor of the result. Of course, low heat also means extra time.
That's where the pressure cooker comes in.
A pressure cooker removes that 212°F limit on moist foods. By trapping steam inside the sealed chamber of a pressure cooker, you create pressure, which in turn raises the boiling point of the liquid.
How's that work? Well, it helps to think of the water in the pressure cooker as a chicken coop from which the chickens are trying to escape. As you add energy to that coop (by, say, pouring some Red Bull into the chicken feed), a few of the overly energized chickens will jump the coop and escape out into the woods. This is the evaporation that occurs at sub-boiling temperatures. Eventually, the chickens in that coop will have enough energy collectively that they'll be able to completely knock down the fence, very rapidly scattering out into the world. This is what happens when water reaches its boiling point.
In a pressure cooker, things are a little bit different. Think of it this way: Instead of allowing those chickens that jump the fence early on to run off into the woods, we retrain them to work for us, holding up the fence and preventing the remaining chickens from escaping. It requires lots more energy to knock down that fence and reach an actual boil.
Because of this, a pressure cooker operating at high pressure (that's 15 pounds per square inch above atmospheric pressure for most models) can achieve non-boiling water temperatures of around 250°F. In this temperature range, sugars will caramelize, and the Maillard reaction will take place quite nicely. Thus, a pressure cooker allows you to increase the rate of caramelization for onions without the risk of accidentally burning them.
I've seen some recipes that call for placing onions inside Mason jars, then placing the jars in a pressure cooker. While this method works, I find it to be almost more tedious than the traditional route, especially considering that it severely limits the volume of onions you can caramelize at once. I prefer to start by melting a few tablespoons of butter in a pressure cooker, then add a few pounds of thinly sliced onions along with a little salt and pepper and a small touch of baking soda. (Baking soda raises the pH of the mixture, which speeds up the rate of the Maillard reaction.) I let the onions soften just enough to begin releasing juices, then cover up the pressure cooker and heat it to high pressure, adjusting the flame as necessary to keep it there. (You want to maintain high pressure without heating it so much that it starts to vent steam. An electric pressure cooker makes this very easy.)*
With some thin-bottomed pressure cookers, you may experience scorching in the onions. If you use a particularly thin pressure cooker, add a cup of water to the pot before sealing it to prevent this.
After 20 minutes at high pressure, I rapidly release the steam, then crack the cooker open. What you end up with inside is onions that are very soft, a rich brown, and extremely sweet-smelling. The stuff is good, but we're not quite done yet.
In order to get maximum caramelized flavor, we want to now reduce what's left of the water in the mixture while continuously stirring, so that we can get those last bits of caramelization that occur above 250°F. It takes just a couple of minutes for the onions to go from golden brown to rich, deep, truly caramelized dark brown.
And here's the good news: This stuff is great. I mean, as-good-as-slow-cooked-caramelized-onions great.
It's not just a quick, cheaty approximation of caramelized onions, like what you get with other stovetop methods—it's actual caramelized onions, ready to be piled on your burger, stuffed into your grilled cheese, added to your stews or sauces or gravies, spooned over your steak. From this stage, you can follow Daniel's classic French onion soup recipe: Just add some sherry, some stock, and a few aromatics, and give it a short simmer. Finish it off with toast and grated Gruyère cheese for full-on Paris bistro flavor with only a few minutes of actual work.
No, we aren't really breaking the unbreakable laws of the universe here, but I won't say anything if you tell people that we are.