In a large skillet over medium heat, brown the Italian sausage until fully cooked. Drain excess fat and transfer the sausage to the slow cooker.
Add the diced onion and minced garlic to the slow cooker with the sausage. Stir in the crushed tomatoes, tomato sauce, diced tomatoes, chicken broth, Italian seasoning, salt, and black pepper. Mix well.
Cover and cook on low for 4 hours or high for 2 hours.
About 30 minutes before serving, stir in the cheese tortellini and heavy cream. Cover and continue to cook on low for 30 minutes or high for 15 minutes, until the tortellini is tender.
Before serving, stir in the shredded mozzarella and grated Parmesan cheese until melted and well combined. Garnish with fresh basil if desired.