Preheat the oven to 350°F with a rack in the center position.
Bring a large pot of salted water to a boil over high heat. Add the pasta and cook according to the package directions to al dente. Drain.
Meanwhile, add the sausage, ground beef, onion, and garlic to a large pot or Dutch oven. Cook over medium-high heat, breaking up the meat with a wooden spoon, until the meat is cooked through and the onions are softened, about 10 minutes.
Add the marinara, crushed tomatoes, salt, Italian seasoning, and butter. Cook, stirring often, until combined and the butter is melted, about 3 minutes. Add the pasta and stir to combine.
Transfer the pasta and meat sauce to a 9 x 13-inch baking dish. Evenly sprinkle with mozzarella, cover with foil, and bake for 25 minutes. Remove the foil and bake for 10 to 15 minutes, until the cheese is lightly browned and the sauce is bubbling. Let cool for 5 to 10 minutes.