In a skillet over medium heat, cook the Italian sausage with the diced white onion and Italian seasoning until the sausage is completely cooked through and the onion is tender.
Add to the slow cooker.
Transfer the cooked sausage and onion to your crockpot. Add the chicken broth, heavy whipping cream, and tomato paste. Stir until combined.
Slow cook.
Cover and cook on LOW for 6–8 hours or HIGH for 4–6 hours.
Add the tortellini, Parmesan & spinach.
Toward the end of cooking, once the soup is hot, stir in the tortellini, Parmesan cheese, and spinach.
Finish Cooking
Continue cooking until the tortellini is tender and the Parmesan has melted into the creamy broth.
Serve warm.
Ladle into bowls and top with additional Parmesan cheese if desired.