Peel and wash the onion and potatoes. Cut potatoes and onion into thin rounds.

Pour olive oil into a 9-inch frying pan and heat it on medium until hot. Toss the onion in and let it cook for about 2 minutes.

Add the potatoes, fry for 8-12 minutes. Salt them with 1 teaspoon of salt. They should be soft and slightly golden.

Crack the eggs into a large bowl, add 1 teaspoon of salt, and whisk together.

Once the potatoes are cooked, remove them from the oil with a slotted spoon, draining any excess oil, and place the potatoes into the bowl of beaten eggs. Mix to evenly coat the potatoes in the egg.

Drain the oil from the pan and save it for another day. Pour a splash of oil into the pan for frying the Spanish omelette. Heat it on medium-high heat.

Once the oil is hot, pour the potato and egg mixture into the pan and even it out with a spatula. Cook for 2-3 minutes or until the surface of the tortilla starts to harden and the edges start to separate from the pan.

Flip the tortilla by placing a plate on top of the pan, and turn it onto the plate with one fast movement. Slide the tortilla back into the pan and let it cook for another 1-2 minutes, shaping the tortilla edges with a spatula or slotted spoon.

Serve the tortilla by sliding it onto a plate or cook on low until the egg is as done as you wish. Enjoy with a side of salad, bread, and your favorite condiments!