1. Brown the Italian sausage in a skillet over medium heat until cooked through, then drain excess fat.

2. Add chopped onion to the skillet and cook until translucent, about 3 to 4 minutes.

3. Stir in minced garlic and cook for 1 minute until fragrant.

4. Transfer the sausage, onion, and garlic to the slow cooker.

5. Pour in chicken broth and add Italian seasoning, red pepper flakes, salt, and black pepper; stir to combine.

6. Cover and cook on low for 6 hours or on high for 3 to 4 hours.

7. About 30 minutes before serving, stir in the cheese tortellini and heavy cream.

8. Add fresh spinach and cook until wilted.

9. Adjust seasoning as needed.

10. Ladle soup into bowls and top with grated Parmesan cheese before serving.