In a mixing bowl, combine ground beef, breadcrumbs, chopped onion, egg, garlic powder, Worcestershire sauce, salt, and pepper. Mix until fully combined.
Form the mixture into meatballs about 2 inches in diameter. You should get around 12-15 meatballs.
In a large skillet over medium heat, brown the meatballs on all sides for about 5-7 minutes. This step is optional but will add extra flavor.
Place the browned meatballs into the slow cooker.
In the same skillet, add the beef broth, sliced mushrooms, and onion soup mix. Stir well and bring to a simmer.
Pour the broth 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.
In the last hour of cooking, mix cornstarch and water in a small bowl to create a slurry. Stir it into the gravy in the slow cooker to thicken it.
Once done, serve the meatballs with the gravy over cooked rice or mashed potatoes. Garnish with fresh parsley if desired.