Preheat oven to 350°F. Cook mostaccioli 1 minute less than package directions. Drain but do not rinse.
Cook Italian sausage, ground beef, and onion in a large skillet until no pink remains and onion is soft. Add garlic and cook 1 minute. Drain excess fat.
Pour in heavy cream and simmer 5 minutes. Add diced tomatoes, tomato paste, and Italian seasoning. Simmer 5 minutes more until thickened.
In a bowl, mix ricotta, 1/2 cup Parmesan, goat cheese, egg, salt, and pepper.
Stir drained pasta into meat sauce.
In greased 9x13 dishes, layer pasta and sauce, cheese mixture, then remaining pasta and sauce.
Bake uncovered for 25 minutes, until bubbling.
Top with cubed mozzarella and remaining Parmesan. Bake 5 more minutes.
Let rest 5 minutes before serving.
Notes
Undercook the pasta by 1 minute so it finishes in the oven. Heavy cream rounds out the tomato sauce and helps it cling to the pasta. Add fresh mozzarella near the end so it melts without overcooking. Let the baked pasta rest before serving for cleaner portions.