Put 3 tablespoons of olive oil in the pan on the hob on a medium heat.
Peel the onion and cut it in half, then cut the halves into thin slices and put them in the pan.
Peel the potatoes, cut them into thin round slices, dry them in a clean tea towel and add them to the pan.
Toss the onion and potatoes in the oil to make sure they are thoroughly coated, and then turn the heat right down, put a lid on the pan and cook them gently for about 20 minutes, or until tender.
Break the eggs into a bowl and lightly whisk them with a fork. When the onions and potatoes are cooked, pour on the egg mixture and add the rest of the olive oil. Keep the heat low and leave the omelette to cook gently for about 20 minutes. When there is no liquid egg left on the top, it’s ready to cook the other side.
Do this either by turning it out upside down onto a plate, and then sliding it back into the frying pan, or by putting the pan under a hot grill for about 5 minutes, or until it’s nicely cooked and golden.
Cut into thin wedges or finger-food sized cubes before serving.
The easiest way to cook tortilla is in a shallow, heavy bottomed pan that can be used on the hob and also in the oven under the grill. This means that you don’t have to turn the tortilla over to cook it on both sides. Otherwise, use an ordinary frying pan, about 20cm in diameter.
Tortilla is delicious hot or cold, and if there are adults sharing the meal, they can add seasoning at the table.