Cook the onion with a bit of olive oil and a pinch of salt on medium heat until they're soft and translucent. Reserve.
Place the potatoes on the same pan cover completely with olive oil. Cook on low heat; you should see just a few bubbles coming up. Let cook for about 20-30min or until you can put a toothpick through them easily.
Take out the potatoes with a slotted spoon and let the potatoes cool to room temperature. At this point you can store the potatoes and onions in the fridge to make the tortilla that evening or the next day.
Pass the olive oil through a sieve and keep on the side to cook the tortilla and then you can use it to pan fry onions, chicken, or something else you cook in the future.
Beat the egg with a teaspoon of salt and mix carefully with the onion and potatoes so as not to break them up too much.
Heat a 20cm diameter non-stick pan (important!) to medium heat with a bit of the olive oil from the potatoes and pour half the tortilla mix in. Let it cook for 2 min (or until golden underneath) and flip it onto a plate. Then it goes back onto the pan with the uncooked side underneath. Watch the video above for more details.
It always fumbles a bit as it goes back into the pan so help it go back into shape using a spatula.
After 2 min (or until golden) take out the tortilla and place it on your presentation plate.