Heat a large pot over medium-low heat. Cook sausage and onion until onion is soft and translucent, breaking up sausage as it cooks, 5 to 7 minutes. Add garlic and cook 1 to 2 minutes more.

Pour all chicken broth into the pot and bring to a boil, about 5 minutes. Add tortellini and cook 5 minutes more. Add tomatoes with juice and return to a boil. Reduce heat to low and simmer soup for 10 minutes.

Stir spinach into soup; it will look like too much, but it will wilt quickly. Add basil and red pepper flakes and simmer 5 minutes more. Season with salt and pepper.

Pass Parmesan cheese at the table when you serve the soup.