Drain tuna well. Clean, wash and drain the salad. Lettuce, except for a few leaves, pluck smaller. Spread the whole salad on four plates
Mix lemon juice, salt, pepper and sugar. Finally, fold in the oil. Clean, wash, quarter and seed the tomato. Finely dice the flesh and add
Pluck the tuna into large pieces with a fork and sprinkle with the marinade. Leave to stand for about 5 minutes. Peel onions and cut them into fine rings
Arrange the tuna pieces with the lemon marinade on the salad. Spread onion rings on top and garnish with a few slices of lemon as desired
Drink: Wine spritzer