Wash and drain the salad. Drain the tuna. Peel garlic and press it through a garlic press. Mix with lemon juice and mayonnaise. Peel onions and cut them into fine rings.
Clean, wash and slice the tomatoes. Cut Ciabatta crosswise into 4 pieces and slice horizontally. Cover the lower bread halves with lettuce and spread with aioli. Cover with tuna, onions and tomatoes and sprinkle with salt and pepper.
Place the top halves of bread on top.