In a mixing bowl, combine the ground beef, breadcrumbs, chopped onion, milk, egg, garlic powder, Worcestershire sauce, salt, and pepper. Mix until well combined.
Form the mixture into meatballs, about 1.5 inches in diameter. This should yield approximately 18-20 meatballs.
In a skillet over medium heat, brown the meatballs on all sides for added flavor (optional).
Place the meatballs in the slow cooker.
In another bowl, mix together the beef broth and mushroom soup. Pour the mixture over the meatballs in the slow cooker.
Cover and cook on low for 6 hours, or until the meatballs are cooked through and tender.
If you desire a thicker gravy, mix cornstarch with a little water to create a slurry and add it to the slow cooker during the last 30 minutes of cooking. Stir gently and let it cook until thickened.
Once cooking is complete, serve the meatballs with the gravy, garnished with fresh parsley if desired.