Peel and finely dice the onion and garlic. Wash and clean the tomatoes and cut them into small cubes. Heat oil in a pot. Sauté onion and garlic in it. Add canned tomatoes, bring to the boil and puree. Add stock and diced tomatoes and let it boil down for about 5 minutes. Cook the pasta in boiling salted water for about 10 minutes.
Wash the basil. Cut leaves into fine strips, except for a few for garnishing. Drain the mozzarella. Season sauce with salt, pepper and sugar. Add mozzarella and basil to the sauce just before serving. Drain the pasta on a sieve and arrange on plates with the sauce. Serve garnished with basil