Peel the garlic and onion, dice very finely and mix with the oil in a bowl. Quarter, core and finely dice tomatoes. Cut the basil into fine strips. Mix everything with the oil and season well with salt and pepper.
Cook the spaghettini in salted water. Slice the parmesan into fine shavings with a peeler. Drain the pasta and mix it dripping wet and hot with the fresh tomato sauce. Sprinkle with parmesan and black olives and serve immediately.