01 - Combine breadcrumbs and milk in a large bowl; allow to soak for 5 minutes.
02 - Add ground beef, ground pork, finely chopped onion, minced garlic, egg, salt, black pepper, allspice, and nutmeg. Gently mix by hand or with a wooden spoon until just incorporated, taking care not to overwork the mixture.
03 - Shape the mixture into small, uniform balls, each approximately the size of a walnut (about 1 1/4 inches in diameter).
04 - Heat butter and vegetable oil in a large skillet over medium heat. Cook the meatballs in batches, turning occasionally, until evenly browned on all sides, roughly 8 minutes per batch. Transfer meatballs to a plate and set aside.
05 - Melt unsalted butter in the same skillet over medium-low heat. Sprinkle in flour and cook while stirring continuously for 1 minute to form a roux.
06 - Gradually add the beef or vegetable stock, whisking constantly to prevent lumps. Stir in heavy cream, soy sauce, and Dijon mustard. Simmer the mixture until the sauce thickens, about 5 to 7 minutes.
07 - Return the browned meatballs to the skillet, gently turning to coat them in the sauce. Simmer over low heat for 5 to 10 minutes, ensuring they are heated through.
08 - Serve immediately, ideally accompanied by mashed potatoes, lingonberry jam, and seasonal steamed vegetables.