This great recipe comes from the amazing Dorie Greenspan and her cookbook Around My French Table. Once you make it once, you’ll figure out how to easily swap out the ingredients with whatever you have and whatever you like.
In a skillet 9 to 10 inches (preferably cast iron or one that can go in the oven), pour 2 tablespoons of oil into skillet and warm over medium heat. Add potatoes and onion, garlic and rosemary. Season with salt and pepper, lower the heat, cook slowly until the potatoes are coked through, about 20 minutes. Discard the garlic and rosemary. Transfer onion and potatoes to a bowl. Wipe out skillet with a paper towel. You want it clean so the tortilla will be easy to unmold.
Position an oven rack so that the pan will ultimately be 6 inches from the heat source. Turn on the broiler now.
In a large bowl, beat the eggs with salt and pepper and the piment d’Espellette or cayenne. Stir in the potatoes and onion. Put the skillet over medium-high heat and pour in about 1 1/2 tablespoons oil. When the oil is hot, add the eggs and potatoes. Immediately lower the heat and let the eggs cook undisturbed for about 2 minutes. Run a silicone spatula or knife around the edges of the pan and cook slowly for another 8 to 10 minutes, or until the top is almost set.
Slide the pan under the broil and cook until the top of the tortilla is set. Check it after one minute, and then keep checking. Transfer to a serving platter and cool before cutting and serving.