In a small bowl, combine panko breadcrumbs and milk; let sit for 5 minutes until liquid is absorbed.
In a large mixing bowl, combine ground beef, ground pork, grated onion, garlic, egg, allspice, nutmeg, salt, pepper, and the soaked breadcrumbs.
Mix by hand until just combined; do not overwork the meat.
Shape the mixture into 24 uniform meatballs, approximately 2.5cm in diameter.
Heat 20g of butter in a large skillet over medium-high heat. Add meatballs and brown on all sides for 8-10 minutes until cooked through. Remove meatballs and set aside.
In the same skillet, melt the remaining 20g of butter. Whisk in the flour and cook for 1-2 minutes until a light brown roux forms.
Slowly whisk in the beef broth and heavy cream, scraping the bottom of the pan for browned bits. Simmer for 3-5 minutes until the sauce thickens.
Return the meatballs to the skillet and toss to coat in the cream sauce.
Serve immediately with a side of lingonberry jam and mashed potatoes or egg noodles.