Peel the potatoes and cut into thin slices about 3 mm thick. Peel and thinly slice the onion. Put both in a 22–24 cm non-stick frying pan with the olive oil and ½ teaspoon salt.

Cook over a low to medium-low heat for 20–25 minutes, turning gently and regularly, until the potatoes are tender but not crisp. Lower the heat if they colour quickly. Carefully drain through a heatproof sieve into a heatproof bowl, reserving the oil.

Beat the eggs in a large bowl with the remaining salt and the pepper. Let the drained potatoes cool for 5 minutes, then fold them and the onion into the eggs without crushing everything.

Return 15 ml of the reserved oil to the pan over a low heat. Add the egg and potato mixture and level the top. Cook for 8–10 minutes until the edges are set and the base is golden; shake the pan gently to check it releases.

Take the pan off the heat. Wearing oven gloves, hold a large flat plate firmly over the pan and carefully invert the tortilla onto it, keeping hot oil away from you. Slide it back into the pan and cook the other side over a low heat for 5–8 minutes, until set throughout with no liquid egg in the centre.

Slide onto a clean plate and rest for 5 minutes before cutting into four wedges. Serve warm as part of a tapas meal.