A well-made Spanish tortilla is so good, so satisfying in every way, that I would nominate it to the Great Food Hall of Fame, if only there were such a thing. It has nothing to do with Mexican flour or corn tortillas. If it has a relative, it would be the Italian frittata.
In Spain, the dish goes by two names: tortilla de patatas or tortilla española. The one thing I avoid calling it is a potato omelet (its English translation) since a Spanish tortilla is more about potatoes than eggs, and the word omelet doesn't really conjure up the right image. Besides, a tortilla is more robust and gratifying than any omelet—and infinitely more versatile. It tastes great whether served warm, cool, or at room temperature. It makes an excellent breakfast, lunch, dinner, snack, or crowd-pleasing tapa. (A tortilla is a sure hit at any party or pot luck.) It can be made ahead, it's reasonably fast cooking (45 minutes, start to finish), and it welcomes variations (see More ways to enjoy a tortilla). At my house, we rarely go more than a week without having one for supper. Taste the magnificent tortilla española for yourself, and you'll know why.
I learned how to make tortillas from my Spanish husband, Isidro, who can stroll into the kitchen, peel and slice some potatoes, chop about half as many onions, lightly beat five or six eggs, and with no apparent thought, turn out a perfect tortilla every time. He insists there's nothing to it, and I agree, now that I'm clued in to the few tricks he'd been keeping secret. Here's how to turn five fairly pedestrian ingredients—potatoes, eggs, onions, olive oil, and salt—into a dish that deserves more fame and glory than any other, except perhaps paella (yes, there's a theme here).
Low-starch potatoes work best
Whether you use a knife or mandoline, the goal is thin, even potato slices, which will cook quickly and at the same rate.
I've used everything from small red potatoes to oblong Idaho russets in tortillas. Any potato will do the job, but I like boiling potatoes, red potatoes, and Yukon Golds best because they have a lower starch content and don't fall apart during frying. I also prefer their firmer texture.
Aim for thin, consistent slices. If you own a mandoline or a V-slicer, set the thickness to 1/8 inch. Or else use a sharp chef's knife, slicing the potatoes as thinly as possible without making it a slow, laborious chore. Thicker slices not only take longer to cook but also make a dry tortilla.
The potatoes are cooked in a generous amount of oil, but don't worry, most of it stays in the pan. (The oil can be strained and reused.) Although the oil temperature is much lower than it is for deep-frying, you should use an oil with a high enough smoke point. I use plain olive oil (not extra-virgin) or else corn oil.
You'll need a deep, nonstick skillet
A hot pan is as good as nonstick. As long as the pan is very hot when the potatoes, onions, and eggs are poured in, the tortilla will release in one piece.
You can make a tortilla with the barest of kitchen equipment, just a bowl and a skillet. The pan must be deep enough to contain all the potatoes and should preferably have gently sloping sides to give the tortilla its shape, which is like a Frisbee. For the recipe that follows, a 10-1/2-inch skillet that's at least 1-1/2 inches deep is ideal. The sliced potatoes will fill the pan, which is fine as long as you turn them carefully as they cook in the oil. Though I'm not usually a fan of nonstick skillets, I do embrace them for tortillas. A tortilla that won't release cleanly from the pan isn't a total disaster, but it is irritating, and messy.
Heat the oil until a potato slice sizzles but doesn't brown. You're not making french fries—you're cooking potatoes until they're tender inside yet soft and pale outside. If a few slices do get golden and crispy, it's no big deal (actually, they're delicious and quite tempting to eat), but remember that this isn't the point. The chopped onions cook the same way. If the pan is deep enough, you can speed things along by cooking the onions and potatoes together, adding the onions to the pan when the potatoes are about halfway done.
This isn't deep-frying. The potatoes will sizzle and the oil will bubble, but the potatoes shouldn't get brown or crisp.
Once the potatoes and onions are cooked and drained, they're added to the beaten eggs. Some Spaniards let the egg and potato mixture sit for a short time, maybe 15 minutes, so the potatoes absorb some of the eggs. I don't find that necessary, but it's okay to do it, if you want. Other cooks crush the potatoes a bit as they sit in the eggs. That is not okay, in my opinion, as it ruins the layered effect that you get in the finished tortilla.
The egg, potato, and onion mixture gets cooked in the same pan that you used to fry the potatoes and onions. Here's where a little knowledge goes a long way.
Wipe out the skillet. If it's not nonstick, use a spatula to scrape out any stuck-on bits, and then wipe out the pan with a wadded paper towel.
To prevent sticking, heat the skillet on high. In a hot pan, the eggs coagulate immediately, before they have time to fill the tiny pores in the pan and stick to it. It doesn't matter how much oil you add to the pan -- if it isn't hot enough when the eggs go in, the tortilla won't come out in one piece.
After the mixture cooks for a minute, reduce the heat. This ensures that the inside sets before the outside burns. A low temperature also seems to make the eggs firmer and denser, which is what you want in a tortilla.
Pick a large, flat plate for flipping
The dramatic climax of tortilla making comes when the eggs have mostly set and the tortilla is ready to be flipped. This is what makes a tortilla different from an Italian frittata (where the pan goes in the oven to finish cooking the eggs).
When the eggs are mostly cooked, set a flat, rimless plate over the pan. Don't forget to give the pan a good shake to confirm that the tortilla is loose.
Give the pan a good shake to release the tortilla. If it isn't loose in the pan, help it along with a spatula. The eggs will be a little jiggly and wet in the center, but the tortilla should slide around as a whole unit.
Find a flat plate that's at least as wide as the pan and has no rim. (In Spain, there exists a special plate whose sole purpose is to flip tortillas—what devotion.) To do the flip, you'll invert the tortilla onto the plate and then slide it back into the pan to finish cooking.
Holding the plate firmly in place, invert the pan so the tortilla falls onto the plate.
Slide the tortilla back into the skillet, pushing any stray potatoes underneath. Tuck around the perimeter to round and neaten the edges.
You can eat the tortilla right away, or chill it and have it for lunch the next day (a wedge on a baguette is how it's usually done). To my taste, a tortilla hits its peak an hour or so after cooking.