In a large mixing bowl, combine ground beef, ground pork, breadcrumbs, beaten egg, salt, pepper, allspice, and chopped onion. Mix well.
Form the mixture into small meatballs, about 1 inch in diameter.
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 remaining butter and drippings. Cook for 1-2 minutes, stirring constantly.
Gradually whisk in beef broth and heavy cream. Cook until the gravy thickens.
Pour the gravy over the meatballs in the slow cooker.
Cover and cook on low for 4 hours.
Serve the meatballs with gravy over mashed potatoes or egg noodles.