Wash the tomatoes and cut into thin slices. Peel and finely chop the onion, wash and chop the basil and oregano.
For the marinade, mix balsamic vinegar with olive oil, salt, pepper, sugar and herbs, adding a little water if necessary.
Put the chopped tomatoes in a bowl with the chopped onion and mix with the marinade. Let the tomato salad stand for a short time before serving.