Toss together, your ground beef, bread crumbs, egg whites, Worcestershire sauce, 1 cup of thinly diced onions, and garlic. Mix it really, whether with your hands, or with a potato masher like I use.
Form into 2" balls. Set aside.
In your slow cooker mix together BBQ sauce of your choice, beef broth, and the remaining onions.
Drop your meatballs into the sauce, give them a gentle stir to make sure they are fully covered in sauce.
Cook on low for 3-4 hours.