In a large bowl, mix together ground beef, ground pork, breadcrumbs, milk, egg, onion, salt, pepper, allspice, and nutmeg.
Form the mixture into small meatballs.
In a skillet, melt butter over medium heat. Add meatballs and cook until browned on all sides.
Transfer meatballs to the slow cooker.
In the same skillet, add flour to the drippings and cook for 1 minute.
Gradually whisk in beef broth until smooth. Bring to a boil, then reduce heat and simmer until thickened.
Pour the gravy over the meatballs in the slow cooker.
Cover and cook on low for 4 hours.
Stir in sour cream before serving.