Heat oil in a large pan, add diced onion. Saute until onions are soft.Crumble Italian Sausage and garlic with the onion. Cook until sausage is no longer pink.Stir in diced tomatoes, tomato sauce, chicken broth, basil, salt and pepper.Bring to a boil, reduce heat to low, and cover.Let soup simmer for 20-30 minutes.Add tortellini and spinach and simmer for about ten minutes, until pasta is tender.Serve with shredded Parmesan.