Heat the oil in a pan. Sauté the onion and garlic. Add the tomato puree and cook briefly. Add the tomatoes and all the other ingredients up to and including the pepper, simmer uncovered for approx. 10 mins. Add the basil.
Cook the spaghetti in boiling salted water until just al dente, set aside approx. 100 ml of the cooking water, drain the spaghetti, add to the sauce along with the reserved cooking water, mix. Transfer to a platter, shape into nests.
Top the spaghetti with the tomatoes and mozzarella, garnish with basil.