Heat the oil in a large skillet over medium-high heat. Add the garlic, bell pepper, and onion. Sauté for about 2 minutes, until slightly softened but still with some bite. Transfer to a bowl and set aside.

Add the sausage to the skillet and cook, breaking it into crumbles, until cooked through and no longer pink. Remove from the heat and drain any excess grease.

Bring a large pot of water to a boil. Add the rigatoni and cook according to package directions until al dente. Drain and return the pasta to the pot.

Reserve a small amount of pepperoni, olives, mushrooms, and sautéed vegetables for topping. Add the remaining pepperoni, olives, mushrooms, and vegetables to the pasta with the marinara, sausage, Italian seasoning, salt, pepper, and 1 cup of cheese. Stir to combine.

Transfer the mixture to a 9×13-inch baking dish and spread evenly.

Sprinkle the remaining 2 cups of cheese over the casserole. Arrange the reserved pepperoni, mushrooms, olives, and vegetables on top.

Bake for 25 minutes, until the cheese is melted and lightly browned and the edges are bubbling. Remove from the oven and serve.