Drain the cheese, cut in half and hollow out generously from the flat side with a ball cutter. Cut the hollowed out into small pieces.
Wash the tomatoes, cut in half, remove the stalk and cut into cubes. Peel and dice the onion as well. Finely chop the basil leaves.
Mix the vinegar with pepper and salt. Stir in the olive oil. Mix the cheese pieces, diced tomatoes, diced onions and basil with the marinade and pour into the mozzarella halves.