Preheat your oven to 350°F (175°C). Bring a large pot of salted water to a boil. Add the rigatoni and cook until it is al dente, usually 1 to 2 minutes less than the package directions state. Drain the pasta well.
While the pasta cooks, place a large Dutch oven or heavy pot over medium-high heat. Add the Italian sausage, ground beef, and diced onion. Cook, breaking the meat apart with a spoon, for about 10 minutes, until the meat is browned and the onion has softened.
Cook until the ground meat reaches an internal temperature of 160°F (71°C). Carefully drain off any excess fat from the pot.
Reduce the heat to medium. Stir in the minced garlic and cook for 30 seconds until fragrant. Add the marinara sauce, crushed tomatoes, kosher salt, Italian seasoning, and butter. Stir until the butter has melted and the sauce is well combined.
Add the drained pasta to the pot with the meat sauce. Stir gently to coat all the pasta evenly.
Spoon the pasta and sauce mixture into a 9x13-inch baking dish and spread it into an even layer. Sprinkle the shredded mozzarella cheese evenly over the top.
Cover the baking dish with aluminum foil. Bake for 25 minutes. Then, remove the foil and continue to bake for another 10 to 15 minutes, or until the cheese is melted and lightly browned and the sauce is bubbling at the edges.
Let the baked rigatoni rest for 5 to 10 minutes before serving. This helps it set up and makes it easier to serve. Garnish with fresh parsley.