Peel and wash the potatoes, then cut them into thumb-sized cubes; rinse and salt lightly.
Fry the potato cubes in hot olive oil, stirring occasionally, until tender but not too browned; drain.
Sauté the finely chopped onion in the same oil until translucent, then add it to the potatoes and pour off the excess oil.
Beat the eggs thoroughly in a bowl with a pinch of salt, then stir in the fried potatoes and onion.
Heat a little oil in a large pan over medium heat, pour in the mixture and cook for 7-8 minutes, gently lifting the edge to check that it isn't sticking.
Flip the tortilla using a large plate, then cook the other side for another 2 minutes before serving.