Begin by preheating the oven to 350°F and adjusting the oven rack to the center position.

Bring a large pot of salted water to a rapid boil over high heat. Add the rigatoni and cook according to the package directions until al dente—firm to the bite. Once cooked, drain the pasta thoroughly and set aside.

In a separate large pot or a heavy-bottomed Dutch oven, set over medium-high heat, combine the ground Italian sausage, lean ground beef, diced onion, and minced garlic. Cook the mixture, stirring and breaking the meat apart with a wooden spoon, until the meat is browned and fully cooked and the onions are soft and translucent, about 10 minutes.

Pour in the marinara sauce and crushed tomatoes. Stir in the kosher salt, Italian seasoning, and butter. Simmer the mixture for approximately 3 minutes, stirring frequently, until the butter melts completely and the sauce becomes thick and cohesive.

Add the cooked rigatoni into the pot and mix well until each piece of pasta is evenly coated in the meat sauce.

Carefully spoon the pasta and sauce mixture into a 9x13-inch baking dish. Evenly scatter the grated mozzarella over the top. Cover the dish tightly with aluminum foil and place it in the preheated oven. Bake for 25 minutes.

Remove the foil and return the dish to the oven. Continue baking for an additional 10 to 15 minutes, or until the cheese is melted, lightly golden, and the sauce is bubbling around the edges.

Allow the baked rigatoni to rest for 5 to 10 minutes before serving. Finish with a generous sprinkle of freshly chopped parsley for a burst of color and freshness.