Peel the potatoes and cut into thin slices. Chop the onions into small pieces. Heat half of the oil in a pan with a high rim (approx. 20 cm in diameter). Cook the potatoes and onions in it for 25 minutes (medium heat). Season with salt and pepper.
Whisk eggs in a bowl, season with salt and pepper. Add the cooled vegetables.
Heat the remaining oil, add the egg mixture and let it set for approx. 6 minutes (medium heat). Keep shaking the pan.
Cover the pan with a flat plate, hold it tightly and turn it over so that the tortilla is on the plate. Carefully slide this back into the pan and let it set again for 6-8 minutes.
Take out the tortilla and cut into pieces of cake.
Tastes warm or cold. Tomato salad goes well with it.