Finely chop the carrot, celery, and onion. Sauté the onion for a few minutes, then add the rest and cook until softened. Add the tomato purée and blend everything with a mixer. Continue cooking over low heat for 5 minutes.

While the rice is still warm, shape it into balls, placing pieces of mozzarella in the center of each. (It's important to do this step while the rice is warm. If the rice sticks to your hands, rub a little oil on your hands.)

Coat each ball first in flour, then in beaten egg, and finally in breadcrumbs. Fry until golden brown.