Preheat oven to 375°F (190°C). Lightly grease a 9x13 inch baking dish.
In a large skillet, cook the Italian sausage and pepperoni over medium-high heat until browned. Drain off any excess grease.
Add the chopped onion, minced garlic, bell peppers, and mushrooms to the skillet. Cook until the vegetables are softened, about 5-7 minutes.
In a medium bowl, combine the ricotta cheese, half of the mozzarella cheese, and half of the Parmesan cheese. Stir in the oregano, basil, garlic powder, salt, and pepper.
Spread the meat and vegetable mixture evenly in the prepared baking dish.
Spoon the pizza sauce over the meat and vegetable layer.
Dollop the ricotta cheese mixture evenly over the pizza sauce.
Sprinkle the remaining mozzarella and Parmesan cheese over the top.
Bake for 25-30 minutes, or until the cheese is melted and bubbly and the casserole is heated through.
Let stand for a few minutes before serving.
Notes
Serve hot with your favorite pizza toppings or a side salad.