Cook the risoni in salted water according to the packet instructions until al dente, drain and reserve 50 ml of the cooking water.
Rinse the pasta and leave to cool. Peel and finely dice the onion and garlic. Chop the basil leaves. Halve the tomatoes. Drain the mozzarella.
Whisk the oil, vinegar and reserved pasta cooking water in a bowl and season with salt and pepper.
Stir in the pasta, onion, garlic, basil, tomatoes and mozzarella.