Here's a challenge for you: Name one dish that can be eaten as a snack, an appetizer, and a main course; that's equally delicious both hot and cold; that almost anyone would be happy to eat at any time of day or night; that is just as perfect for your kid's lunchbox as it is alongside a stiff drink; that's considered as much the domain of home cooks as of restaurant chefs; and that's composed of only a few very ordinary ingredients.
Pizza comes close, but, as good as cold pizza is, let's face it—it's better hot. I've been thinking about this for several days now, and the only good answer I can come up with is Spain's famed tortilla española, also sometimes called tortilla de patatas.
Made with egg, potato, olive oil, and sometimes onion, it's one of those rare dishes that demonstrate how a handful of household ingredients can be transformed from ho-hum to exceptional with just a little bit of good technique. It may seem hard to get jazzed about an egg and potato omelette, but, when made well, tortilla española is one of the greatest of egg dishes. No wonder the Spanish find just about any excuse to eat it.
The hard part, as one might expect when dealing with a national dish made by millions of cooks in Spain and abroad, is that there are many ideas about what a tortilla española should be. In both my travels and my recipe research, I've seen ones as thin as pancakes and others as thick as cheese wheels. I've seen some made with onions and some without. I've seen the potatoes fried vigorously until crisp and browned—Ferran Adrià famously put potato chips in his—and, alternatively, cooked gently to the point of falling apart. Once they're cut open, I've seen some tortillas ooze loose egg and others that are solid all the way through. Clearly, there's no one right way to do it.
This, then, is the way I like it: nice and thick, with onions, the potatoes cooked slowly in oil until they're silky and softened, and the egg moist and custardy but not runny in the center.
Here's what you need to know to make it at home.
Step 1: Salt Your Eggs
This is not the first step you'll see in most tortilla española recipes, but there's a good reason to change that. Contrary to popular belief, pre-salting eggs doesn't thin them and turn them watery; it actually helps them retain moisture during cooking (more on the science of that here). Once I've added a generous pinch of salt and beaten the eggs very well, I set them aside while I get to work on the rest of the tortilla.
Step 2: Cook Your Potatoes and, Yes, Onions
Next, peel and slice the potatoes and onions thinly. I like Yukon Golds for their smoother, less starchy texture, but you can use russets, too. The onions are a contentious topic among Spanish cooks, and some will swear that including them invalidates the recipe. ("It's tortilla de patatas, not patatas y cebollas," they'll say.) That may be, but if so, an onion-y tortilla española is the most delicious version of wrong I've ever tasted. Slowly cooked in the oil with the potatoes, the onions are melting and sweet, but they also serve another important function: They add moisture to the tortilla. I like to think of them as an insurance policy against dryness. Even if you accidentally cook the tortilla a little more than you intended, those pockets of moisture guarantee it won't taste that way.
Now comes the most extravagant part of the recipe: frying in oil. It's extravagant because you need a lot of oil here—my recipe calls for about two cups, and it's 100% olive oil, which isn't exactly cheap. Some modern recipes for home cooks use significantly less oil, but it just doesn't come out the same. I like to go the traditional route here.
I have a few things to say in defense of this. First, it's essential to bathe the potatoes and onions in ample oil so that everything tenderizes evenly and so that oil flavor can work its way into all of the vegetables. Second, you should feel free to use a less expensive olive oil here: Heating it kills off most of the nuanced flavors of the pricier oils anyway. And third, you end up draining the cooked potatoes and onions, and the leftover oil is even more delicious than before; it can be used again in other dishes (or to fry subsequent batches of potato and onion for more tortillas, which you will want to make). Ultimately, none of it goes to waste.
I heat the oil until it's shimmering, then add all of the potatoes and onions to the skillet. At first, they won't all seem to fit, but with a little stirring, they'll compress as they cook. I keep it just hot enough that the oil is bubbling around all the pieces of food, but not so hot that it's frying vigorously. Once done, the potatoes will be extremely tender and breaking apart, and the onions will be smooth as silk. This takes about 20 minutes.
I find that the texture of the cooked potatoes plays a critical role in how the tortilla turns out. When tortilla española is made with firmer slices of potato that retain their shape, what you end up with is an omelette that has a very clear divide between egg and filling. There are sections of egg, and then sections of potato. This approach makes it more difficult to form and flip the tortilla, since the eggy part is so loose and runny. But when the potatoes are partially broken down, they mix into the egg and thicken it. (The residual heat from the potatoes and onions also helps thicken the eggs slightly.)
The tortilla is easier to form and flip with the thicker batter, and the interior has much less of a clear demarcation between potato and egg: Instead, it has a delicious cohesion, with larger bits of potato and onion studded throughout.
Once they're done, I drain the potatoes and onions through a mesh strainer, saving that oil for later use. When they're free of excess oil, I season the potatoes and onions well—a bland potato and egg dish will never be a good one.
Step 3: Froth Eggs, Then Mix in Potato and Onion
I give the eggs one final vigorous beating to froth them up, which helps create an airier tortilla later, and then mix the potato and egg in.
Some people swear that you should let the potato-egg mixture sit for a half hour or more before cooking, but I don't have the patience for that, and I haven't noticed any major difference in batches left to sit longer. (In fact, letting the potato mixture cool too much can make it harder to cook the tortilla, as the cold mixture will take longer to set in the center of a thick tortilla.) I give it about five minutes or so, which is more or less how long it takes to clean out the skillet—a 10-inch one, for my recipe—and heat a few tablespoons of the reserved oil in it.
A note on the skillet: Traditionally, this would all be done in a carbon steel skillet, and, if you have one that's well seasoned, you certainly can use it here for both the frying and the omelette-cooking steps. If not, nonstick is your best option. I use the nonstick for both steps. (The potatoes and eggs will ensure that the nonstick surface never reaches dangerous vaporization temperatures, even if you place the pan over high heat.)
Step 4: Cook, Shake, Swirl
When the oil in the skillet is hot, I scrape the egg mixture in, shaking and swirling the pan to keep the tortilla in motion. Heat regulation is important here: Too hot, and the surface may burn before the egg has cooked through; not hot enough, and you won't get much browning.
Step 5: Form
As soon as the egg is noticeably thicker, I take a spatula and push it in all around the sides. The idea is to start giving the tortilla its characteristic puck-like shape. The more solid the egg gets, the more it will hold that shape. Ultimately, though, the key to getting the shape right is to flip the tortilla.
Step 6: Flip
Flipping is the part about making a tortilla española that most people dread, but it's not too hard, and it's essential for getting just the right shape and texture. Skipping the flip and using the broiler to cook the top of the tortilla has a tendency to puff it up and create a very different texture from the one you're after here.
Exactly when you flip, and how many times you do it, is a matter of personal preference. If you want a runny center, you'll need to attempt a flip while the egg is still fairly loose in the center. If you want it well done, you can wait until the egg is more set. I try to take a middle road, with the center and top still slightly fluid, which should yield a custardy center when it's finished.
For those who are uncomfortable with the flip, doing it just once may be more than enough. But if you have the technique down, flipping back and forth a few times can be helpful in creating a creamier center from edge to edge: Much like with flipping a steak, the more times you turn the tortilla, the more the heat building up near the surface can dissipate, preventing those sub-surface layers from cooking as much as they otherwise would. If you feel up to the task, flipping more is a good idea.
To do it, you'll want to set a plate with a shallow (or no) rim, a flat lid, or some other firm, flat surface upside down on top of the tortilla; it can be larger than the pan by a few inches, but it must, at a minimum, be slightly larger than the tortilla itself. Then lift the skillet up, place your hand flat on top of the plate, and, in one very quick motion, rotate so the plate is on the bottom. The faster you do this, the better, since hesitation and slowness are exactly the things that will lead to your tortilla slipping out and hitting the floor. I'd recommend doing this over the sink or a garbage can just in case of any drips. Holding the plate or lid with a dish towel is also advisable. (Remember: That tortilla is hot!)
After you've successfully flipped it, add another tablespoon or two of reserved oil to the pan, then carefully slide the tortilla back into it. Use the spatula again to press in on the sides and reinforce that puck shape.
At this point, you need to cook the tortilla only long enough to set the bottom (again, flipping it one or two more times, if you wish, until it's done). Leave it a little longer if you want a more fully cooked center (called "cuajada" in Spanish) or less if you want it runny ("jugosa" or "babosa," as they say).
Then you can either carefully slide the tortilla out of the skillet onto a serving plate, or flip it out onto a serving plate using the same technique as before.
How you eat it—hot, cold, in small cubes as an hors d'oeuvre, in a hefty wedge for dinner—is entirely up to you (though, however you serve it, I'd recommend a generous dollop of homemade allioli on the side). The options are endless.