Preheat your oven to 350°F (175°C) and position the rack in the center. Grease a 9x13-inch baking dish with nonstick cooking spray and set it aside.

Bring a large pot of salted water to a boil over high heat. Once boiling, add the mostaccioli pasta and cook according to the package instructions until al dente. Drain the pasta thoroughly and set it aside.

Heat a large skillet or Dutch oven over medium-high heat. Add the Italian sausage and ground beef, cooking until browned. Use a wooden spoon to break the meat into smaller pieces as it cooks. Add the diced onion and minced garlic to the skillet, stirring frequently until the onion is softened and fragrant, approximately 8-10 minutes.

Stir the marinara sauce, crushed tomatoes, kosher salt, Italian seasoning, and heavy cream into the skillet. Mix well until the ingredients are fully combined. Allow the mixture to simmer for 3-5 minutes, letting the flavors meld together.

Add the drained mostaccioli pasta to the sauce, stirring gently to coat every piece evenly. Ensure the pasta is thoroughly mixed with the sauce for the best flavor distribution.

Transfer the pasta mixture to the prepared 9x13-inch baking dish, spreading it evenly. Sprinkle the shredded mozzarella cheese over the top, covering the pasta completely. Cover the dish loosely with aluminum foil to prevent sticking.

Place the baking dish in the preheated oven and bake for 25 minutes. Remove the foil and continue baking for an additional 15 minutes, or until the cheese is melted, bubbly, and lightly golden on top.

Remove the dish from the oven and let it rest for 5-10 minutes. Sprinkle freshly minced parsley over the top for a pop of color and flavor. Serve warm and enjoy.