Preheat Oven & Prep Pan: Preheat oven to 350°F (177°C). Coat a 9x13 inch baking dish with cooking spray. Set aside.
Brown the Meat: Heat olive oil in a large skillet or Dutch oven over medium high heat. Add ground beef and Italian sausage. Season with salt and pepper. Cook for 3-4 minutes, breaking up the meat with a spoon.
Add Onion: Add the chopped onion to the pan. Cook for an additional 5 minutes, until meat is browned and cooked through and onion is softened.
Add Garlic & Sauces: Stir in minced garlic and cook for 30 seconds until fragrant. Add marinara sauce and tomato sauce. Bring to a simmer.
Simmer Sauce: Let the sauce simmer for 5 minutes, until slightly thickened. Taste and adjust seasoning with more salt and pepper if desired.
Combine Pasta and Sauce: Add the cooked and drained rigatoni to the skillet with the sauce. Toss well to coat evenly.
Assemble Dish: Pour the pasta mixture into the prepared baking dish and spread into an even layer. Sprinkle shredded mozzarella cheese evenly over the top.
Bake Covered: Cover the dish with aluminum foil. Bake for 20 minutes.
Bake Uncovered: Remove the foil and bake for another 15-20 minutes, until the sauce is bubbly and the cheese is melted and golden brown.
Rest & Serve: Remove from oven and sprinkle with chopped parsley. Let stand for 5 minutes before serving.