Preheat oven to 375°F (190°C). Grease a 9×13-inch baking dish.
In a large bowl, combine ground beef, ground pork (if using), breadcrumbs, milk, egg, salt, pepper, allspice, nutmeg, and chopped onion. Mix well and shape into 1-inch meatballs.
In a large skillet, melt butter over medium-high heat. Brown meatballs in batches until cooked through, about 5-7 minutes. Transfer to a plate.
In the same skillet, whisk in flour and cook for 1 minute. Gradually whisk in beef broth, heavy cream, and Worcestershire sauce. Simmer until thickened, about 5 minutes. Season with salt and pepper.
Cook pasta according to package instructions until al dente. Drain and set aside.
Add cooked pasta and meatballs to the skillet with the sauce. Toss to coat evenly.
Transfer mixture to the prepared baking dish. Sprinkle with shredded mozzarella cheese.
Cover with foil and bake for 20 minutes. Remove foil and bake for an additional 10 minutes, until cheese is melted and bubbly.
Garnish with chopped parsley before serving. Enjoy hot!