Preheat your oven to 375°F (190°C) and lightly spray a 9x13 inch baking dish with cooking spray.

In a large pot, bring salted water to a rolling boil, add the rotini pasta and cook according to package instructions (8-10 minutes). Drain pasta in a colander but do not rinse.

Heat a large skillet over medium-high heat, add ground beef with Italian seasoning, garlic powder, and kosher salt; cook for 5-7 minutes until browned. Drain excess grease.

In a mixing bowl, combine the drained rotini, cooked beef, chopped pepperoni, diced green bell pepper, onion, marinara sauce, and 2 cups of shredded mozzarella cheese. Stir until well-mixed.

Spoon the mixture into the prepared baking dish, spreading out evenly. Bake for 15 minutes until bubbly.

Sprinkle the remaining mozzarella over the top and arrange whole pepperoni slices. Return to the oven for an additional 10-15 minutes, or until the cheese is melted and golden brown.

Let the casserole sit for a few minutes before serving. Enjoy the comforting taste of your homemade casserole!