In a small bowl, soak the breadcrumbs in milk for 5 minutes until soft.
In a large bowl, combine the soaked breadcrumbs, chopped onion, ground beef, ground pork, beaten egg, salt, pepper, and allspice. Mix gently until just combined.
Shape the mixture into small meatballs, about 1 inch (2.5 cm) in diameter.
Heat butter and olive oil in a large skillet over medium heat. Add the meatballs in batches and cook until browned on all sides and cooked through, about 7-8 minutes. Remove meatballs and set aside.
In the same skillet, sprinkle the flour over the fat and whisk to form a roux. Cook for 1-2 minutes, stirring constantly.
Gradually whisk in beef broth, scraping up any browned bits from the pan. Bring to a simmer.
Stir in heavy cream and Worcestershire sauce. Simmer until the sauce thickens, about 5 minutes.
Return the meatballs to the skillet and gently coat them in the sauce. Cook together for another 5 minutes to combine flavors.
Garnish with chopped parsley if desired and serve hot with mashed potatoes or egg noodles.