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.
Traditionally, you brown a tortilla on both sides by flipping it halfway through cooking on the stovetop. I am not a traditionalist in this regard. It is many times easier simply to bake the whole thing in the oven, where that residual fat will find its way to the top of your tortilla and give it a gorgeous brown sheen. (Pour off any oil that pools on the top of the tortilla before serving.)
In another regard, I am a traditionalist. A Spanish tortilla should contain peeled potatoes, onion, eggs, olive oil, and salt and pepper -- nothing else. Ultra-orthodox types might challenge me on the pepper, which does interfere with the otherwise uniformly cream-hued interior of the tortilla. But eggs and black pepper are irresistibly harmonious, like Tegan and Sara. Don't fight it.
Yield: 8 servings. Time: About 1 hour, partially unattended
Extra-virgin olive oil for braising
1 1/2 pounds waxy potatoes, peeled and thinly sliced
1 medium yellow onion, thinly sliced
Salt and black pepper
10 large eggs
Heat the oven to 375°F. Put 3/4 inch of olive oil in a large pot over medium heat. When it's hot, add the potatoes and onion, season with salt and pepper, and stir well to coat with the oil. Cook, stirring occasionally, until the potatoes and onion are very tender, 30 to 40 minutes.
Meanwhile, break the eggs into a medium bowl, season with salt and pepper, and beat vigorously for about 1 minute. Put a large skillet over medium heat; when it's hot, transfer the potatoes and onion from the pot to the skillet with a spatula or slotted spoon, leaving most of the oil behind. (Reserve the oil for another use.) Pour the eggs over the potatoes and onion, stir very gently just to distribute all the ingredients evenly in the skillet, and bake until golden brown, about 20 minutes. Serve warm, at room temperature, or chilled. (Store leftover tortilla wrapped in foil in the refrigerator for up to a few days.)