01 - Combine breadcrumbs and milk in a large bowl. Allow to stand for 5 minutes to absorb.
02 - Add ground beef, ground pork, onion, garlic, egg, salt, pepper, allspice, and nutmeg to the breadcrumbs. Mix gently until just combined, avoiding excess handling.
03 - Shape the mixture into walnut-sized balls approximately 1 1/4 inches in diameter, ensuring uniform sizing for even cooking.
04 - Heat butter and oil in a large skillet over medium heat. Cook meatballs in batches, turning occasionally, until evenly browned on all sides, about 8 minutes per batch. Transfer to a plate and set aside.
05 - Melt butter in the same skillet. Whisk in flour, stirring constantly, and cook for 1 minute to form a light roux.
06 - Gradually add stock while whisking to prevent lumps. Stir in heavy cream, soy sauce, and Dijon mustard. Simmer over medium heat until thickened, about 5 to 7 minutes. Season with salt and pepper to taste.
07 - Return the meatballs to the skillet, coating them thoroughly in the sauce. Simmer gently for 5 to 10 minutes, ensuring they are heated through.
08 - Plate meatballs with sauce. Recommended accompaniments include mashed potatoes, lingonberry jam, and steamed vegetables. Serve hot.