Follow these simple steps to whip up your Pizza Pasta Casserole:

1. Preheat Oven: Begin by preheating your oven to 375°F (190°C).
2. Cook Pasta: In a large pot, bring salted water to a boil. Cook the pasta according to package instructions until al dente. Drain and set aside.
3. Sauté Meat: In a large skillet, heat the olive oil over medium heat. Add the ground beef or sausage, and cook until browned, breaking it apart with a spoon.
4. Add Aromatics: Once the meat is cooked, add the diced onion and minced garlic. Sauté until the onion is translucent.
5. Mix Sauce: Stir in the marinara sauce, Italian seasoning, and red pepper flakes. Allow it to simmer for a few minutes until all ingredients are well combined.
6. Combine Pasta: In a large mixing bowl, combine the cooked pasta with the meat and sauce mixture.
7. Layer Ingredients: In a large baking dish, pour half of the pasta mixture. Sprinkle half of the mozzarella cheese and layer half of the pepperoni slices. Repeat with the remaining pasta, cheese, and pepperoni.
8. Bake: Cover the baking dish with foil and bake for 25 minutes. Remove the foil and return to the oven for an additional 10-15 minutes until the cheese is bubbly and golden.
9. Cool Down: Once done, take the casserole out of the oven and let it sit for 5-10 minutes before serving.
10. Garnish: If desired, sprinkle fresh basil or parsley on top for a pop of color and flavor.

By following these steps, you will create an amazing Pizza Pasta Casserole that is rich in flavor and easy to make.