The word tortilla is a diminutive form of torta, one of several Spanish words for “cake.” Torta means different things in different countries — and this is true of its diminutive form as well. In Mexico and Central America, tortilla typically refers to the cornmeal- or wheat-flour-based flatbread Americans know and love. In Spain, it more commonly refers to a frittata-like dish of eggs and potatoes. The latter, unlike the former, can legitimately be described as a savory cake: It’s thick, rich, and indulgent.
I would never say that the Spanish tortilla is superior to the Mexican tortilla: Each is important and necessary in its own way. But I do want to suggest that maybe, when we Americans hear the word “tortilla,” our minds should not automatically jump to the flatbread. Because we are missing out.
Consider the tortilla española, often translated as “Spanish omelette,” an English rendering that is wildly inaccurate and also does an injustice to both omelettes and tortillas. An omelette is fluffy, pliable, and ideally pale rather than browned. A Spanish tortilla is none of those things. Even to describe it as a potato frittata, as I did a couple of paragraphs ago, is to miss the mark slightly. Like a frittata, a tortilla is dense, substantial, and intended to be cut into wedges. But a tortilla is a much more specific thing than a potato frittata: You can’t merely boil some potatoes, pour some eggs over them, cook the thing, and call it a day. A tortilla is not a tortilla unless it’s made with potatoes that have been braised slowly in lots and lots of olive oil. This is not the same as frying — your goal is to cook the potatoes (with onions) until they’re extremely tender, not to crisp them up.
Once that’s done, most of the oil gets drained away. (You should absolutely save this onion- and potato-infused oil in the fridge and use it in other savory dishes — your roasted vegetables will be sublime for weeks.) But this draining doesn’t mean a tortilla is a low-fat dish. You’re not trying to rid the cooked potatoes of oil entirely before you add the eggs; don’t, God forbid, blot away excess oil with paper towels or anything like that. You need some of that residual fat to help crisp up and brown the surface of the tortilla.