This tortilla is distinguished by its density because it’s got a high potato-to-egg ratio. Flip it often for an even texture. This is best at room temperature, so definitely make it ahead.
1. Heat the vegetable oil in a medium skillet over medium-high heat. Add all of the potatoes—the oil should come to the top of the potatoes (add a bit more if it doesn’t). Once everything starts to sizzle, turn the heat down to medium and sprinkle it with a teaspoon of salt. Cook the potatoes for ten minutes, stirring constantly. The potatoes shouldn’t be browning, just getting cooked through. Add the onions and cook for ten more minutes. It’s okay if the vegetables turn a little golden at this point. Remove the potatoes and onions with a slotted spoon to a paper towel-lined plate and let them sit for five minutes.
2. Whisk the eggs together in a large bowl with 1/2 teaspoon of salt. Add the potatoes and onions and gently fold everything together. Heat the olive oil in a small (about 7″ diameter) nonstick skillet over medium heat. Add the egg mixture and cook for 1 1/2 minutes or until the bottom is just set. Using a plate, invert the tortilla and return it to the pan. Continue to flip it using a plate every 30 seconds for a total of six minutes (a dozen flips). At the end, the whole thing will be set and browned and lovely. Let cool to room temperature before serving.
Originally featured in Spanish Recipes