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.