Peel, rinse and cut the potatoes into thin slices. Peel the onion and chop finely.
Heat half of the olive oil in a pan and cook the potato slices and chopped onion in it over medium heat for 25 minutes. Season with salt and pepper.
Beat the eggs in a bowl, and season with salt and pepper. Cool the potato mixture slightly and add into the beaten eggs.
To prepare the tortillas, heat the remaining olive oil in a pan and pour some egg-potato mixture into it. Fry the tortilla for 6 minutes and flip with the help of a plate. Fry on the other side for 6 minutes. Drain the tortilla on kitchen paper. Cook more tortillas until the mixture is used up.
Cut the tortilla into pieces and arrange on plates.