Step 1: Preheat your oven to 375°F (190°C). Lightly grease a 9x13 inch baking dish.
Step 2: In a large skillet, heat the olive oil over medium-high heat. Add the ground Italian sausage and chopped onion. Cook, breaking up the sausage with a spoon, until browned and the onion is softened, about 5-7 minutes. Drain any excess fat.
Step 3: Add the minced garlic, dried oregano, and red pepper flakes (if using) to the skillet. Cook for 1 minute more until fragrant.
Step 4: Stir in the marinara or pizza sauce, bringing it to a gentle simmer. Remove from heat.
Step 5: Pour half of the sauce mixture into the prepared baking dish. Spread it evenly. Layer the uncooked tortellini over the sauce in a single layer.
Step 6: Top the tortellini with the remaining sauce mixture. Sprinkle 1.5 cups of the shredded mozzarella and half of the Parmesan cheese over the sauce.
Step 7: Arrange the pepperoni slices evenly over the cheese. Sprinkle with the remaining 1/2 cup mozzarella and the rest of the Parmesan cheese.
Step 8: Cover the baking dish tightly with aluminum foil and bake for 20 minutes.
Step 9: Remove the foil and bake for an additional 5-10 minutes, or until the cheese is melted, bubbly, and lightly golden. Let stand for 5 minutes before serving.