Clean and wash the celery, put the leaves aside and cut the rest into thin slices.
For the dressing, mix together the mustard, vinegar, salt, pepper and sugar, fold in the oil. Mix with the celery and leave covered for about half an hour.
Wash the tomatoes, cut in halves or quarters depending on the size, cutting out the stalk base. Add to the marinated celery together with the halved mozzarella balls, mix, season to taste. Serve sprinkled with the chopped celery leaves.