Heat the oil in a large frying pan or saucepan. Add the onion and allow to soften and brown a little. Add the passata and sugar. Then stir in the chicken, garlic, chilli flakes and seasoning. Bring to a simmer and cook for about ten minutes.
Meanwhile cook the pasta in a large saucepan as per packet instructions. Drain and reserve 100mls of the cooking water. Return the pasta to the saucepan.
Add the spinach to the chicken mixture and allow it to wilt. Add in some of the cooking water if the sauce is too thick. Stir through the lemon juice. Taste and add more seasoning if necessary. Combine the sauce with the pasta. Bring to the table either in one large bowl or individual dishes with the pieces of mozzarella and basil leaves sprinkled over.