Peel, wash and cut the potatoes into thick slices. Heat the oil in a pan. Put the potatoes in the pan and season with 1 teaspoon of salt. Turn the potatoes until they are all covered with oil and fry over medium heat for about 10 minutes.
Peel and finely chop the onion. Add to the potatoes and fry for another 10 minutes until the potatoes are golden brown. Beat the eggs and season with salt and pepper. Drain the oil, except for about 2 tablespoons.
Pour egg milk over it and let it simmer covered at low heat until the egg mass is firm. Remove the omelette from the pan, let it rest for a while and cut it into 12 pieces.