Clean, wash and cut the tomatoes into small pieces. Peel onions, cut them into very fine rings. For the marinade, peel the garlic and press it through a garlic press. Wash the parsley, pluck the leaves from the stalks and, except for a few for garnishing, chop them. Mix vinegar with salt, pepper, sugar and garlic, add oil.
Stir in chopped parsley. Mix the tomato and onions with the marinade and arrange on plates. Slice the cheese on top. Garnish with parsley leaves