Brown the sausage in a skillet and drain off any excess grease.
Transfer the browned sausage to a slow cooker.
Add the onion, garlic, chicken broth, diced tomatoes, Italian seasoning, salt, and pepper to the slow cooker.
Cook on low for 6-8 hours or on high for 3-4 hours.
During the last 30 minutes of cooking, add the tortellini and spinach.
Stir in the heavy cream and cook until heated through.
Serve immediately.