Preheat the oven to 350°F. Lightly grease a 2.5-qt baking dish. Set aside.
Cook the pasta according to the package directions. Drain and set aside.
Cook and crumble the Italian sausage and onion in a large skillet over medium heat until the sausage is evenly cooked and the onion is tender. Drain the grease.
Mix the sausage and onion, marinara, cream, pepperoni, ham, Italian seasoning, and garlic powder in a very large bowl. Add the cooked pasta and toss until evenly coated. Transfer to the prepared baking dish.
Cover and bake the casserole for 30 minutes. Remove the cover, top with the cheeses, and continue baking for about 15 minutes, or until the cheese is lightly browned. Sprinkle the fresh parsley on top, if desired.
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