In a large bowl, combine ground beef, breadcrumbs, milk, chopped onion, egg, garlic powder, Worcestershire sauce, salt, and pepper. Mix until well blended.
Form the mixture into meatballs, about 1 inch in diameter.
(Optional) In a large skillet over medium heat, brown the meatballs for about 4-5 minutes, turning them to brown all sides.
Place the meatballs in the slow cooker.
In a separate bowl, mix together the beef broth, sliced mushrooms, and onion soup mix. Pour the mixture over the meatballs in the slow cooker.
Cover and cook on low for about 6 hours or until meatballs are cooked through and tender.
(Optional) If you want a thicker gravy, mix the cornstarch with water to create a slurry. Stir it into the slow cooker in the last 30 minutes of cooking and allow it to thicken.
Serve the meatballs over mashed potatoes, rice, or egg noodles for a complete meal.