In a large bowl, combine ground beef, ground pork, breadcrumbs, beaten egg, salt, and pepper. Mix well.
Shape the mixture into meatballs.
In a skillet, melt butter over medium heat and cook onions until soft.
Add the meatballs to the skillet and brown on all sides.
Transfer the meatballs to the slow cooker.
In a bowl, whisk together beef broth, heavy cream, flour, Worcestershire sauce, salt, and pepper. Pour over the meatballs in the slow cooker.
Cover and cook on low for 4 hours.
Serve the meatballs with the gravy over mashed potatoes or egg noodles.