Clean, wash and cut the salad into strips. Peel onions, cut 2 into rings, dice the rest finely. Wash, quarter, seed and dice the tomato. Sage, except for a few leaves, cut into strips.
Mix vinegar with salt, pepper and sugar. Whip the oil into it. Add diced tomatoes, onion and sage strips. Arrange salad, onion rings and Parma ham on plates. Cut mozzarella into 8 slices.
Place sage leaves on top and place on a baking tray lined with baking paper. Bake briefly under the hot grill of the oven. Place 2 mozzarella slices on each salad plate and place the tomato vinaigrette on top.
Sprinkle with pepper.