Cook the pasta in plenty of boiling salted water for about 10 minutes. Quench and let cool down.
Peel and finely chop the onion. Mix with 6 tbsp vinegar, salt, pepper and sugar. Fold in the oil. Mix marinade and noodles.
Drain the mozzarella. Wash the tomatoes and basil. Put some basil aside for garnishing, cut the rest into strips. Cut the tomatoes in half. Mix everything with the noodles and season again.