### Instructions
1. **Preheat and Cook Tortellini:** Preheat your oven to 375°F (190°C). Bring a large pot of lightly salted water to a boil. Add the tortellini and cook according to package directions, typically 2-3 minutes, until al dente. Drain well and set aside.
2. **Sauté Sausage and Aromatics:** While the tortellini cooks, heat olive oil in a large oven-safe skillet or Dutch oven over medium-high heat. Add the ground Italian sausage and cook, breaking it up with a spoon, until browned and cooked through. Drain any excess grease.
3. **Add Vegetables and Seasoning:** Stir in the chopped onion and cook until softened, about 3-5 minutes. Add the minced garlic and cook for another minute until fragrant.
4. **Combine with Sauce:** Pour in the marinara sauce, Italian seasoning, and red pepper flakes (if using). Bring to a simmer, then reduce heat to low and cook for 5 minutes, allowing the flavors to meld.
5. **Assemble and Bake:** Stir the cooked tortellini into the sauce mixture until well coated. Sprinkle evenly with mozzarella cheese and Parmesan cheese.
6. **Bake to Perfection:** Transfer the skillet to the preheated oven (if not oven-safe, transfer to a 9×13 inch baking dish). Bake for 15-20 minutes, or until the cheese is melted and bubbly and the casserole is heated through.
7. **Garnish and Serve:** If desired, sprinkle with mini pepperoni, olives, bell peppers, or fresh basil. Let the casserole rest for a few minutes before serving.
### Short Tips
* **Prep Ahead:** You can cook the sausage and sauce mixture a day in advance and store it in the refrigerator. When ready to bake, just add the cooked tortellini and cheese, then proceed with baking.
* **Veggie Boost:** Feel free to add other quick-cooking vegetables to the sauce, such as spinach (stir in at the end until wilted) or diced bell peppers (sauté with the onion).
* **Freezer-Friendly:** This casserole freezes well! Assemble the casserole (without baking), cover tightly with foil, and freeze for up to 3 months. Thaw overnight in the refrigerator, then bake as directed, adding an extra 10-15 minutes to the baking time.
* **Cheese Please:** Don’t be shy with the cheese! A mix of mozzarella and provolone, or even a sprinkle of ricotta, can elevate the cheesiness.
