01 - Combine breadcrumbs and milk in a large bowl and let stand for 5 minutes to hydrate.
02 - Add ground beef, ground pork, finely chopped onion, minced garlic, egg, salt, black pepper, allspice, and nutmeg to the bowl. Mix by hand just until ingredients are incorporated; take care not to overwork the mixture.
03 - Roll the mixture into small, walnut-sized balls, approximately 1.25 inches in diameter.
04 - In a large skillet over medium heat, add butter and vegetable oil. Working in batches, fry meatballs for about 8 minutes, turning as needed until evenly browned. Remove from skillet and set aside.
05 - Melt butter in the same skillet over medium heat. Sprinkle in flour and whisk for one minute until a roux forms.
06 - Gradually pour in stock while whisking to blend, then add heavy cream, soy sauce, and Dijon mustard. Stir and simmer until the sauce thickens, about 5 to 7 minutes.
07 - Return browned meatballs to the skillet, coating them evenly with the sauce. Simmer gently for 5 to 10 minutes until the meatballs are heated through and the sauce is creamy.
08 - Serve meatballs hot, accompanied by mashed potatoes, lingonberry jam, and steamed seasonal vegetables as desired.