Brown the Italian sausage in a skillet over medium heat, breaking it into crumbles as it cooks. Add the diced onion and minced garlic during the last few minutes.
Transfer the sausage mixture to the slow cooker. Pour in the chicken broth and stir in the Italian seasoning, salt, and black pepper.
Cover and cook on low for 6 hours.
Stir in the tortellini and spinach. Continue cooking on low for 20 more minutes until the tortellini is tender.
Turn off the heat and slowly stir in the heavy cream until fully combined.
Ladle into bowls and top each serving with grated Parmesan cheese.