Wash the potatoes and cook in boiling water for about 20 minutes. Then rinse with cold water, peel and leave to rest for a while. Peel the onions. Halve one onion and cut into strips.
Drain the tuna. Cut the potatoes into slices. Heat oil in a pan (approx. 24 cm Ø). Fry the potatoes and onion strips in it at high heat for about 5 minutes. Season with salt and pepper.
Pluck the tuna into small pieces, add 2/3 of it, mix in. Whisk eggs, season with salt and pepper. Pour evenly over the potato pan. Leave to stand on a low heat for 15-20 minutes.
Shake the pan from time to time so that the tortilla does not touch the bottom. Slide the tortilla onto a plate. Place the tortilla back in the pan with the browned side up and fry for another 3-4 minutes.
Chop one onion roughly. Mix with the rest of the tuna and spread it on the tortilla. Pluck the lettuce leaves, wash and drain well. Pluck leaves smaller. Dice 1 onion very finely. Mix with lemon juice.
Season with salt, pepper and sugar. Mix with the salad leaves. Serve tortilla and salad in portions.