Peel and wash the potatoes and cut them into slices of about 1/2 cm thickness. Heat 4 tablespoons of oil in a large pan and fry the potato slices in it for about 15 minutes until golden brown. Peel and finely dice the onions and add them to the potatoes 5 minutes before the end of the cooking time.
Season with salt and pepper. Whisk eggs, salt and pour over potatoes. Leave to set over a low heat for approx. 15-20 minutes, shaking the pan from time to time so that the tortilla does not touch the bottom.
Turn the tortilla over onto a plate. Heat the rest of the oil in the pan. Slide the tortilla into the pan with the browned side facing upwards. Fry for another 3-4 minutes.