When most Americans in the U.S. hear "tortilla," they think of the flat Mexican bread.
When most Spanish folks – Europeans not Mexicans – hear "tortilla," they think of a kind of omelet that contains just three ingredients: potato, onion, and egg. The word "tortilla" means, literally, little cake, and the Spanish tortilla, when made correctly, is like a little cake,
I'm not a fan of breakfast-for-dinner. The practice of having bacon and eggs or, good god, pancakes and suchlike in the evening is disorienting and unpleasant. Breakfast-all-day might work for McDonald's – actually the sales of breakfast items for lunch and dinner seems to be working out quite well for the company – but breakfast-all-day, or at any time of day after about 10 am, doesn't work for me. The cool thing about the Spanish tortilla is that, like a quiche, it's got basically breakfast ingredients reconfigured for any time of day. It doesn't feel weird to eat a Spanish tortilla after the sun goes down, even with a glass of wine, which is somehow unthinkable with bacon and eggs (not that it hasn't, of course, been done).
It's an easy recipe, with just the three ingredients. You thinly slice potatoes and pan fry in a good amount of oil; when the potatoes are maybe 6 or so minutes from being fork-tender, add roughly cut onions and warm to transparency. When both potatoes and onions are tender, remove from heat and add to a mixture of eggs. All ratios are up to you and variable: I used two smallish potatoes, one medium onion and four eggs for one batch; a dozen small potatoes, one medium onion and five eggs for another; it's all good, and this is a very flexible and forgiving recipe.
Put oil in the (ideally non-stick) pan, turn heat to medium-low, and pour in the egg/potato/onion mixture – and don't touch it for about 3-4 minutes, except for using a rubber spatula to loosen the edges. You want the bottom to brown without sticking before you flip it, which is tricky. To flip, use the rubber spatula to loosen the tortilla, then put a rimless pan lid on the top and flip the tortilla onto it so that the already-cooked side is on top; then slide the uncooked side into the pan. Use the rubber spatula to again shape around the sides of the tortilla so that it looks pretty.
And appearance counts. The Spanish tortilla has a certain beige/yellow/white cast that is defeated if you leave the brown or red skins on the potatoes, which would be my tendency. Europeans, if I may generalize, seem to disdain potato skins: years ago, in a Left Bank raclette restaurant, I watched as French people, young and old, carefully peeled each boiled potato before smearing it with raclette cheese. My impression is that Europeans believe leaving the skin on potatoes to be gauche (even on La Rive Gauche), so I peeled the spuds a la mode Européen. For I am a fancy lad.
The potatoes provide some loft, so that the tortilla looks more like a cake than do most omelets. To reinforce the cake-like qualities, tortilla is usually served by slicing out a triangular portion; I guess you could sit down to a whole tortilla and eat it like an omelet, but that seems wrong. Also wrong, to me at least, is adding anything besides salt and perhaps pepper to it – I had some Gochujong Korean chili sauce, which I frequently use on eggs, and it wasn't terrible, but it seemed wrong. There's something immensely satisfying about the time-honored, classic simplicity of this three-ingredient meal. Adding anything more seems out-of-sync, kind of like having breakfast for dinner.