With a small knife, slice off the tomato tops and reserve. Slice a
few millimeters (1/16 of an inch) off the bottom so that the
tomatoes can sit flat on a plate; discard the bottoms. Carve out
the seeds and the flesh and discard. In a bowl, toss together
mozzarella, red onion, basil, vinegar and salt and pepper. Stuff
each tomato with the mozzarella mix. Top with the tomato tops.
Serve and enjoy.