Step 1: Brown the Sausage
1. Cook the Sausage: In a skillet over medium heat, brown the Italian sausage until fully cooked, breaking it apart with a spatula. Drain excess fat and set aside.

Step 2: Prepare the Crockpot
1. Combine Ingredients: In the crockpot, combine the browned sausage, diced onion, minced garlic, crushed tomatoes, tomato sauce, Italian seasoning, red pepper flakes (if using), and broth. Stir to combine.

Step 3: Cook
1. Set the Crockpot: Cover and cook on low for 4–6 hours or on high for 2–3 hours.

Step 4: Add Tortellini
1. Add Tortellini: About 15–20 minutes before serving, stir in the cheese tortellini. Cover and cook until the tortellini is tender (fresh tortellini will take about 10 minutes; frozen may take a bit longer).

Step 5: Finish and Serve
1. Add Cheese: Once the tortellini is cooked, stir in the shredded mozzarella cheese until melted and well combined.
2. Garnish and Serve: Season with salt and pepper to taste. Garnish with fresh basil or parsley if desired. Serve hot.