Lightly grease a 9- x 13-inch baking dish and heat the oven to 350 F.
Bring a pot with 3 to 4 quarts of salted water to a boil over high heat. Add the pasta and cook following the package directions for al dente. Drain and set aside.
While the pasta is cooking, heat the olive oil in a Dutch oven or large sauté pan over medium heat. Add the ground beef or sausage and onion and cook while breaking up and stirring until the onion is translucent. This will take about 5 to 7 minutes. Drain off and discard excess grease.
Add the bell pepper and garlic and continue cooking for 1 to 2 minutes, until the meat is no longer pink.
Add the Italian seasoning, crushed red pepper, pizza sauce, tomatoes, olives, and pepperoni to the ground meat. Heat until the sauce begins to simmer. Taste and add salt and freshly ground black pepper, as needed.
Combine the meat and sauce mixture with the drained pasta. Toss to blend thoroughly.
Spoon half of the pasta mixture into the prepared baking dish. Top with half of the shredded mozzarella cheese. Spoon the remaining pasta mixture over the cheese layer and finish with the remaining cheese.
Bake for 25 to 35 minutes, until the casserole is hot and bubbly and the cheese is thoroughly melted.