Preheat your oven to 375℉. Spray a 9×13 baking dish with cooking spray and set it aside.

Bring a large pot of salted water to a boil. Cook the rotini pasta according to the box directions until al dente, then drain it in a colander without rinsing.

In a large skillet, cook and crumble the ground beef over medium-high heat, seasoning it with Italian seasoning, garlic powder, and kosher salt until no longer pink. Drain any excess grease.

In a large mixing bowl, combine the cooked pasta, beef mixture, chopped pepperoni (save 20 whole slices for the top), diced bell pepper, diced onion, marinara sauce, and 2 cups of mozzarella cheese. Stir until well combined.

Pour the pasta mixture into the prepared casserole pan and spread it evenly. Bake for 15 minutes.

Remove from the oven and sprinkle 1 cup of mozzarella cheese over the top, followed by the reserved 20 whole pepperoni slices. Bake for an additional 10-15 minutes.

Serve hot.