Heat the oil in a non-stick medium-sized frying pan.
Fry potatoes until crispy, golden and tender.
Add the onion and fry until transparent.
Drain oil (use a metal colander if available), until all of the oil is drained.
Return potatoes and onions to the pan, arranging them in a single layer; reduce heat down to low.
In a small bowl, using an electric mixer, beat eggs with salt and pepper to taste.
Pour egg mixture into the pan, moving the potatoes and onions around the eggs using a plastic spatula.
Every so often, move the eggs around the potatoes, while cooking, to ensure the eggs cook on the bottom and there is not much eggs left on the top.
Place a big, round plate on top of the pan; with your hand holding the plate, flip the tortilla onto the plate.
Then, slide the tortilla back into the pan carefully, moving the onion and potatoes carefully back into the pan along with the tortilla.
Keep cooking; gently shaking the pan occasionally, until completely cooked through.
When baked, remove the tortilla from the pan by placing a clean plate on top of the tortilla and flip again onto the plate.
Garnish with parsley, optiona).
Serve with a salad or use as an entree or appetizer.
Traditionally served as tapas in little squares.