In a large bowl, combine the ground beef, ground pork, breadcrumbs, onion, egg, milk, salt, pepper, allspice, and nutmeg. Mix gently with your hands or a fork until just combined. Do not overmix. Form the mixture into 1-inch meatballs and set aside.

Have the beef broth measured and ready. Set out the heavy cream and sour cream so they can be added smoothly once the sauce is made.

Heat butter in a large skillet over medium heat. Add the meatballs in batches and brown on all sides until well colored. Transfer the browned meatballs to a plate. They do not need to be cooked through at this stage.

In the same skillet, melt the butter, scraping up any browned bits. Whisk in the flour and cook until golden and nutty. Slowly whisk in the beef broth until smooth and bring to a gentle simmer. Add the Worcestershire sauce, Dijon mustard, and soy sauce if using, then simmer until slightly thickened.

Reduce the heat to low and whisk in the heavy cream and sour cream until smooth. Season with salt and black pepper to taste. Return the meatballs to the skillet, spoon the sauce over the top, cover, and simmer until the meatballs are fully cooked through. Taste and adjust seasoning if needed. If the sauce becomes too thick, thin it with a splash of broth or milk.

Prepare and brown the meatballs as described above, then transfer them to the slow cooker.

In the same skillet, melt the butter, whisk in the flour, and cook until golden. Slowly whisk in the beef broth until smooth, then add the Worcestershire sauce, Dijon mustard, and soy sauce if using. Bring just to a gentle simmer, then pour the sauce over the meatballs in the slow cooker.

Cover and cook on LOW until the meatballs are tender and fully cooked, or on HIGH for a shorter cooking time. During the last portion of cooking, stir in the heavy cream and sour cream until smooth. Season with salt and black pepper to taste. If needed, loosen the sauce with a splash of warm broth or milk before serving.