Combine 1 lb ground beef, 1/2 lb ground pork, 1/2 cup breadcrumbs, 1 small grated onion, 1 egg, 1/4 cup milk, 1 tsp salt, 1/2 tsp pepper, 1/4 tsp allspice, and 1/4 tsp nutmeg in a bowl. Mix well.
Form the mixture into 1-inch balls. Heat 2 tbsp butter in a large skillet over medium heat. Brown the meatballs in batches until cooked through (6–8 minutes). Set aside.
In the same skillet, add 3 tbsp butter and melt. Stir in 3 tbsp flour and cook for 1 minute. Gradually whisk in 2 cups beef broth, 1 cup heavy cream, 1 tbsp Worcestershire sauce, and 1 tsp Dijon mustard.
Bring the sauce to a simmer, stirring constantly, until it thickens.
Return the meatballs to the skillet, coating them in the sauce. Simmer for an additional 5–7 minutes until heated through.
Adjust salt and pepper if needed.
Plate the meatballs with the creamy sauce over egg noodles, mashed potatoes, or rice. Garnish with fresh parsley.