Sauté the onions, garlic and chilli in olive oil, add the tomato paste and stir in well. Deglaze with a little water and let it boil down, season with salt, pepper and a little sugar. The sauce should be nice and creamy, if necessary, I add a little cooking water from the pasta. Just before the end, add the cherry tomatoes and the finely plucked basil.
In the meantime, cook the spaghettini al dente, drain and add to the sauce. Mix everything well, serve immediately and spread the mozzarella on top.