In a small microwave safe bowl, add onions. Cook in microwave for 2 minutes, to soften.
Coat a large, nonstick pan or pot with cooking spray. Add sausage pieces, cooked onions and garlic. Cook and stir over medium heat for 2 minutes. Stir in all broth and tomatoes. Bring to a boil.
Stir in tortellini and return to a boil. Cook for 5 minutes until pasta is almost tender. Stir often. Reduce heat, mix in spinach, basil, black pepper and pepper flakes. Cook for 2 minutes until spinach is wilted and tortellini is tender.
Ladle into bowls. Top with a little Parmesan cheese, if desired. ThIs soup freezes great, too. Since the pasta absorbs some of the liquid, when reheating, you might need to add a little more chicken broth.