Peel the potatoes and cut into very thin slices using a mandolin (fine kitchen slicer). It is important that the slices are thin, otherwise the cooking process will take too long. Soak the potato slices in cold water for 10 minutes, then strain.
Peel the onion and chop finely.
Heat a pan with 1 tablespoon olive oil, add the onions and simmer for about 8 minutes over low heat, stirring occasionally. Pour into a bowl.
In the same pan, heat gently with the rest of the olive oil and add the potato slices. Simmer over low heat with the lid closed for about 12-15 minutes or until the potatoes are cooked. Stir in between.
Put the potatoes in a bowl. Using a whisk, beat the eggs with the salt vigorously. Add to the potatoes. Also add the fried onions and mix everything together.
Heat the pan again, add a small amount of olive oil and pour in the potato, onion and egg mixture. Fry for about 5–8 minutes, until the sides of the tortilla set. Now the tortilla needs to be turned.
Place a second pan of the same size or slightly larger upside down on the first and turn both together once by 180 degrees so that the side that has not yet been fried lies in the pan and can fry. Alternatively, an oiled plate can be used: place it on the pan, turn the whole thing upside down and let it slide from the plate back into the pan. Let cook for another 5 minutes. Serve warm.