How to cook
Cut the potatoes into thin slices and the onions into round slices using a sharp knife. Heat 2 tablespoons of the olive oil in a deep pan and add the onions and potatoes to braise for 20 min. on low heat until the potatoes soften, all while stirring occasionally to prevent them from sticking.
Clean the pan with paper towels, pour in the remaining olive oil and add the potato mixture. Cook for about 5-8 min, lifting the edges of the tortilla with a spatula from time to time. Shake the pan from time to time so that the tortilla doesn't stick.
Flip the tortilla carefully using a plate and braise it another 2-3 min. until golden brown on both sides. Cut it up and serve it hot or cold, garnished with parsley and tomato slices.