I have long loved the tortillas I get at my favorite Spanish tapas restaurants but had trouble making the dish at home until I hit upon this method of poaching the potatoes and onions in oil in the oven beforehand. Now this is something I make almost every week. The chorizo adds a lot of flavor but the dish is almost as good without. For brunch I would cook the potatoes and onions the night before and finish the tortilla in the morning. —JoyManning
Preheat the oven to 400 degrees. Combine the onions, potatoes, chorizo, olive oil and 1/2 teaspoon of the salt in a large oven-proof skillet (cast iron is best). Cover with aluminum foil and bake for 40 minutes, stirring the contents of the pan halfway through cooking. Transfer this mixture to glass bowl and allow to it cool for 30 minutes to an hour.
Add two tablespoons of olive oil to the skillet, and place it over medium heat. Pour the eggs over the potato mixture, and add the remaining 1/4 teaspoon of salt and several grinds of black pepper. When the pan is hot, pour this mixture into the oil and cook undisturbed for 8 to 10 minutes, until you see the edges of the egg setting up.
Fire up the broiler, and transfer the skillet so that it’s situated about 4 inches below the heating element. Broil until golden brown and puffy, about 5 minutes, depending on your broiler. Allow to cool slightly in pan for at least 10 minutes; cut into wedges, and serve hot or at room temperature.