In a large bowl, combine ground beef, ground pork, breadcrumbs, milk, egg, onion, salt, allspice, and pepper. Mix well with your hands until evenly distributed.
Shape the mixture into small meatballs, about 1 inch in diameter.
Heat 1 tablespoon of butter in a large skillet over medium-high heat. Brown the meatballs in batches, ensuring not to overcrowd the pan. Set aside.
In the same skillet, melt the remaining butter. Whisk in the flour and cook for 1 minute, stirring constantly.
Gradually whisk in the beef broth until smooth. Bring to a simmer, stirring constantly.
Stir in the heavy cream, soy sauce, and Dijon mustard. Season with salt and pepper to taste.
Return the meatballs to the skillet and simmer for 15-20 minutes, or until cooked through and the sauce has thickened.
Serve hot with mashed potatoes or egg noodles and lingonberry jam.
Notes
For a richer flavor, use bone broth. You can also add a splash of Worcestershire sauce to the gravy.