Heat the oven to 375°F. Put ¾ 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.)
