Tortilla de Patatas - Spanish omelette

Peel, wash and dry the potatoes. Cut them into thin slices and place them in a deep frying pan with the olive oil at a medium temperature so that they cook slowly.
Add the finely chopped onion. Fry it slowly with the potatoes until they are soft and begin to brown.  Drain the potato mix and leave to cool slightly.
Beat eggs in a bowl with a little salt. Add the potato mix and mix through. 
Return the pan to the heat with a little of the drained oil and add the mixture. Let it simmer over the frying pan for about 5 or 10 minutes on a medium heat, until golden brown.
Turn the tortilla with the help of a lid (always bigger than the pan) and let it fall on the other side in the pan until slowly set. It has to be golden brown on both sides.