Place the Italian sausage, onion, carrots, celery, and garlic into the slow cooker.
Pour the chicken broth over the ingredients. Stir in the diced tomatoes, Italian seasoning, basil, and pepper.
Cover and cook on low for 6 to 7 hours or on high for 3 to 4 hours, until vegetables are tender.
Stir in the tomato sauce and cubed cream cheese until the cream cheese melts completely into the soup base.
Increase the heat to high if necessary, and add the tortellini. Cook for another 15 to 20 minutes, or until the tortellini are cooked through.
Stir in the heavy cream during the last 5 minutes of cooking, if using.
Ladle the soup into bowls and garnish with fresh parsley before serving.
Notes
For a richer flavor, brown the sausage in a skillet before adding it to the slow cooker. Drain off any excess grease.
If you prefer a thicker soup, mix 2 tablespoons of cornstarch with 1/4 cup of cold water, then stir the slurry into the soup during the last 30 minutes of cooking.