Wash potatoes thoroughly and cook in boiling water for 15-18 minutes. Drain, rinse and peel. Leave to cool a little. Drain tuna well in a sieve. Peel and finely chop the onion.
Drain the olives. Mix the eggs in a bowl briefly with the whisk of the hand mixer. Season well with salt and pepper. Pluck the tuna. Cut potatoes into cubes. Heat olive oil in a pan (bottom 22.5 cm Ø, top 29 cm Ø and 5.5 cm high).
Sauté the potatoes in it. Add onion and fry briefly. Add olives and tuna. Pour eggs on top and let everything simmer for about 20 minutes at low heat. Carefully remove the tortilla from the edge of the pan, let it slide out of the pan and cut it into pieces.
Arrange on small plates.