How to Make It
Preheat the broiler. Heat 1/4 cup of the olive oil in a large cast-iron skillet. Add the potato and onion slices, season with salt and pepper and cook over moderate heat, stirring frequently, until the potatoes and onion are tender but not browned, about 15 minutes.
In a large bowl, lightly beat the eggs and season with salt and pepper. Scrape the potato mixture into the bowl, being sure not to leave any in the skillet.
Return the skillet to the heat and add the remaining 2 tablespoons of oil. Add the egg mixture, spreading it out in an even layer. Cover and cook over low heat until the tortilla is set on the bottom and the edges, about 10 minutes. Transfer the skillet to the oven and broil 8 inches from the heat just until the top is set, about 1 minute longer. Set a large plate over the skillet and carefully invert the tortilla onto the plate. Let stand for 5 minutes. Cut into wedges and serve warm or at room temperature.