Heat the olive oil in a 24 cm non-stick frying pan over medium-low heat. Add the potatoes and onion with a generous pinch of salt. Cook gently, stirring occasionally, for 20–25 minutes until the potatoes are soft but not browned. Drain, reserving the oil.

Beat the eggs in a large bowl with a pinch of salt. Add the warm potato and onion mixture to the eggs. Mix gently and leave to rest for 5 minutes.

Heat 2 tbsp of the reserved oil in the pan over medium heat. Pour in the egg and potato mixture. Cook for 5 minutes until the edges are set but the centre is still slightly liquid.

Place a large plate over the pan and flip the tortilla onto the plate in one swift movement. Slide it back into the pan, uncooked side down.

Cook for a further 2–3 minutes. The tortilla should be set on the outside but still slightly custardy in the centre.

Slide onto a plate. Serve warm, at room temperature or cold, cut into wedges.