Prepare the ingredients: Use the leftovers cold. Chop the fried potatoes and sausage/chorizo, set aside.
Sauté the onion & pepper: In a non-stick pan, fry the onion and pepper in a drizzle of olive oil over medium heat until softened and golden. You can also add some garlic for a deeper taste.
Mix the eggs: In a bowl, whisk eggs with Parmesan, oregano, salt, and pepper. Stir in sausage and potatoes.
Cook the tortilla: Pour the mixture into the pan. Cook over low–medium heat for about 5 minutes, gently stirring until the edges start to set. (For a moister tortilla, whisk in an extra egg before cooking.)
Flip & finish: Place a plate over the pan, flip the tortilla carefully, and slide it back into the pan. Cook for another3–4 minutes, until fully set and golden.
Serve: Let the tortilla rest for 2–3 minutes, then slice. Enjoy with a tomato salad dressed with lemon, oregano, garlic, and extra virgin olive oil.