Step 1: Preheat the Oven
Preheat your oven to 375°F (190°C). This ensures the dish bakes evenly and the cheese melts perfectly.

Step 2: Cook the Tortellini
Bring 4 cups of water to a boil in a large pot. Add 1 tsp of salt and the tortellini. Cook according to the package instructions (usually 7-10 minutes for fresh tortellini or 10-12 minutes for frozen). Drain and set aside.

Step 3: Prepare the Meat Sauce
In a skillet, cook the ground beef and Italian sausage over medium heat until browned. Add the chopped onion and minced garlic, cooking until the onion is translucent. Stir in the pepperoni pieces, tomato sauce, tomato paste, oregano, basil, red pepper flakes, salt, and black pepper. Simmer for 5-7 minutes to let the flavors meld.

Step 4: Combine Tortellini and Meat Sauce
In a large mixing bowl, combine the cooked tortellini and meat sauce. Mix well to ensure the tortellini is evenly coated.

Step 5: Assemble and Bake
Transfer the mixture to a greased 9×13-inch baking dish. Sprinkle the shredded mozzarella and grated Parmesan cheese evenly over the top. Bake in the preheated oven for 20-25 minutes, or until the cheese is bubbly and golden brown.