In a frying pan, warm the oil and add the onion and the potatoes. Allow to cook over a low heat until poached. Drain and save the olive oil. In a bowl, mix the drained potatoes and onions together with the eggs and salt. In a tortilla frying pan, add a small amount of hot oil and return the mixture to the pan. Working with a spatula, give the omelette an oval form. Midway through, turn the omelette over and finish cooking over a low heat.