In a large pot over medium heat, add the Italian sausage, breaking it apart with a spatula. Cook until browned and fully cooked, about 5–7 minutes, stirring occasionally. Once done, drain excess fat.

Add the chopped onion to the pot with the sausage and sauté for about 5 minutes until the onions are translucent and tender. Stir in the minced garlic, cooking for an additional minute until fragrant.

Pour in the reduced-sodium chicken broth and water, stirring to combine. Increase heat to high and bring to a rolling boil. Once boiling, reduce the heat to medium-low and allow it to simmer uncovered for 10 minutes.

Return the pot to a boil, then add the fresh spinach and refrigerated cheese tortellini. Stir gently and cook for about 7-9 minutes, until the tortellini are tender and the spinach is wilted.

Ladle the soup into bowls and sprinkle with Parmesan cheese. If desired, add crushed red pepper flakes for extra kick.