Peel and cut the potatoes in small and thin cubes.
Peel and chop the onion, then add it to the potatoes with a quarter teaspoon of salt, and mix well
In a frying pan, add two tablespoons of oil and fry half the potatoes and onion until golden and crispy.
Remove them from the pan, and fry the second batch of potatoes.
When they are cooked, add the first batch of potatoes back to the pan.
In a bowl, whisk the eggs well with the remaining salt.
Add more oil to the pan if necessary, before adding the beaten eggs.
Cook gently on a low to medium heat until the edges become golden.
Shake the pan a bit, then place a plate large enough to cover the pan on top of it and quickly flip the tortilla over.
Slowing transfer the omelette back to the pan.
Cook on the other side until completely set. Use the back of a spoon to press gently over the tortilla so that the middle can be cooked properly.
Seve hot or cold.