Preheat your oven to 375°F (190°C) and prepare a baking dish by spraying it with cooking spray.

Bring salted water to a rolling boil in a large pot. Add rotini pasta and cook until al dente, about 8-10 minutes. Drain and set aside.

In a skillet, cook ground beef over medium-high heat for 5-7 minutes while breaking apart. Add Italian seasoning, garlic powder, and salt. Drain excess fat.

In a mixing bowl, combine cooked rotini, beef mixture, chopped pepperoni, diced bell pepper, diced onion, marinara sauce, and 2 cups mozzarella cheese. Stir until well combined.

Transfer the mixture to the baking dish and bake for 15 minutes.

Remove the casserole, sprinkle with 1 cup of mozzarella cheese and add reserved pepperoni slices. Bake for another 10-15 minutes until cheese is bubbly and golden.