01 - Combine breadcrumbs and milk in a large bowl and allow to stand for 5 minutes until softened.
02 - Incorporate ground beef, ground pork, chopped onion, minced garlic, egg, salt, black pepper, allspice, and nutmeg into the breadcrumb mixture. Mix gently until just combined, avoiding over-mixing.
03 - Form mixture into small spheres, each approximately the size of a walnut (about 1.2 inches in diameter).
04 - Heat butter and oil in a large skillet over medium heat. Fry meatballs in batches, turning occasionally, until evenly browned all over, about 8 minutes per batch. Remove meatballs and set aside.
05 - Melt butter in the same skillet. Whisk in flour and cook for 1 minute, allowing the roux to develop a golden hue.
06 - Gradually pour in stock while whisking to prevent lumps. Add heavy cream, soy sauce, and Dijon mustard. Season with salt and black pepper. Simmer, stirring, until the sauce thickens, about 5 to 7 minutes.
07 - Return browned meatballs to the skillet, turning to coat with the sauce. Simmer gently for 5 to 10 minutes, ensuring meatballs are heated through.
08 - Arrange hot meatballs with sauce on a serving platter. Accompany with mashed potatoes, lingonberry jam, and steamed vegetables if desired.