Quarter the tomatoes and remove the pulp. Finely dice the tomatoes, garlic, onion and mozzarella. If you like, you can sweat the garlic and onion in a little oil until translucent, but it is not a must.
Mix all the diced ingredients with high-quality olive oil and season with salt and sugar. Finely chop the basil and fold in. The sauce must now steep for at least 1 hour.
Cook the spaghetti in salted water according to the instructions on the packet until al dente. Serve the hot spaghetti with the cold sauce.