In a large bowl, mix ground beef and pork.
Add breadcrumbs, grated onion, milk, egg, salt, pepper, and allspice.
Combine until well mixed, then shape into meatballs.
Heat butter in a large skillet over medium heat.
Add meatballs to the skillet, cooking until browned on all sides.
Remove meatballs and set aside on a plate.
In the same skillet, add beef broth and bring to a simmer.
Stir in heavy cream and return meatballs to the skillet.
Let simmer for about 10 minutes, until the sauce thickens.
Serve hot over mashed potatoes or with lingonberry sauce.