Peel and finely chop the onions and garlic. Peel, wash and chop carrots and celery. Heat 1 tablespoon of oil in a saucepan. Fry the onions, garlic and vegetables in it.
Add the tomatoes with the liquid, broth and bay leaf, bring to the boil and simmer for about 15 minutes. In the meantime cut bread into cubes. Heat 2 tablespoons of oil in a pan. Fry the bread cubes until golden brown.
Mix with pesto. Drain mozzarella and cut into cubes. Remove laurel from the soup. Puree the soup with a chopping stick and season with salt and pepper. Add mozzarella cubes to the soup.
Serve the tomato and mozzarella soup with the croutons.