Clean, wash and cut the salad into bite-sized pieces. Peel onions and cut into rings. Drain the tuna and pluck it into large pieces with a fork. Wash, clean, quarter and seed the tomato.
Cut the fruit flesh into small cubes. Mix vinegar and lemon juice, season with salt and pepper. Whip the oil into it and add the tomato cubes. Arrange lettuce, goat and tuna on four plates, add tomato vinaigrette.