**1. Preheat the Oven:**
- Preheat your oven to 375°F (190°C) and grease a 9x13-inch baking dish.
**2. Cook the Tortellini:**
- Bring a large pot of salted water to a boil. Cook the tortellini according to package instructions until al dente. Drain and set aside.
**3. Cook the Meat Mixture:**
- In a large skillet over medium heat, cook the ground beef or sausage with the diced onion until browned. Add garlic and sauté for 1 minute. Drain any excess grease. Stir in the marinara sauce and season with salt, pepper, Italian seasoning, and red pepper flakes, if using.
**4. Make the Creamy Mixture:**
- In a mixing bowl, combine the ricotta cheese, cream cheese, sour cream, 1/2 cup mozzarella, and Parmesan cheese. Mix until smooth.
**5. Assemble the Casserole:**
- Spread a thin layer of the meat sauce in the bottom of the prepared baking dish. Layer the cooked tortellini over the sauce. Spread the creamy mixture evenly over the tortellini. Top with the remaining meat sauce.
**6. Add Cheese and Bake:**
- Sprinkle the remaining 1/2 cup of mozzarella cheese on top. Cover with foil and bake for 20 minutes. Remove the foil and bake for an additional 10-15 minutes, or until the cheese is bubbly and golden.
**7. Serve:**
- Let the casserole rest for 5 minutes before serving. Garnish with fresh parsley and enjoy!