01 - Combine breadcrumbs and milk in a large bowl and let soak for 5 minutes.
02 - Add ground beef, ground pork, onion, garlic, egg, salt, black pepper, allspice, and nutmeg to the bowl. Gently mix until just combined, avoiding over-mixing.
03 - Roll mixture into balls about 1 1/4 inches in diameter (walnut-sized).
04 - Heat butter and vegetable oil in a large skillet over medium heat. Fry meatballs in batches, turning occasionally, until evenly browned on all sides, approximately 8 minutes. Remove from skillet and set aside.
05 - Melt butter in the same skillet. Whisk in flour and cook for 1 minute until slightly golden.
06 - Gradually pour in stock while whisking to prevent lumps. Add heavy cream, soy sauce, and Dijon mustard. Stir and simmer until sauce is thickened, about 5 to 7 minutes.
07 - Return browned meatballs to the skillet. Toss gently to coat with sauce and simmer gently 5 to 10 minutes until meatballs are cooked through.
08 - Plate meatballs hot with sauce, alongside mashed potatoes, lingonberry jam, and steamed vegetables if desired.