Clean and wash the tomatoes. Remove the stalk base. Cut the tomatoes into slices. Cut melon into slices and remove seeds. Cut the flesh (up to 4 slices) from the skin.
Cut into small pieces. For the marinade, wash the basil and chop finely except for a few leaves for garnishing. Mix vinegar and basil. Season with salt, pepper and sugar. Whip the oil into it.
Halve one melon wedge crosswise and arrange on plates with the remaining prepared ingredients and mortadella. Spread the marinade on top. Serve garnished with basil.