Wash the potatoes still in their skins, place them in a large pot, cover them with cold water, add salt and put on the heat. Once boiling, cook for half an hour until the potatoes are cooked but firm. Drain and leave to cool.
Meanwhile, peel the onions and slice them into julienne strips, not too thin. Place them in a bowl and cover with Chianti Wine Vinegar. Leave to marinate for 15-20 minutes.
Prepare the tuna: cut the fresh tuna steak into slices about 1.5 cm thick, then into cubes. Place in a bowl and season with salt, pepper, oil and lime zest and leave to season for about 15 minutes.
Wash the tomatoes and cut them into pieces. Peel the boiled potatoes and cut them into not too large pieces.
Combine the boiled potatoes, tomatoes and well drained onion in a bowl and season with oil, salt, pepper and a dash of Chianti Wine Vinegar. Add a little lime juice to the tuna left to marinate and combine with the rest of the salad. Decorate with basil leaves.