In a large bowl, combine the beef, beaten egg, worcestershire sauce, salt, garlic powder, onion powder, pepper, and breadcrumbs using your hands, just until combined.Form the beef mixture into about 24 meatballs (about 1 1/2 inches in diameter) and set aside on a large plate.
Heat the olive oil in a large skillet over medium heat.When the oil is hot, add half of the meatballs to the pan and sear on each side for about 2-3 minutes until browned, transfer to the bottom of the slow cooker.Repeat with the remaining meatballs.
Place the frozen meatballs, sliced onion, and mushrooms in the bottom of your slow cooker. Set aside.
In a medium bowl whisk together the beef broth, gravy mix, worcestershire sauce, ketchup, and seasonings. Pour mixture over meatballs. Cook on low for 6-8 hours, or on high for 3-4 hours.
Once cooked, remove the meatballs from the slow cooker and set aside. At this point if your slow cooker was on low, turn it up to high. Mix together the cold water and cornstarch in a small bowl. Whisk the cornstarch mixture into the broth for 1-2 minutes until thickened.
Return the meatballs to the slow cooker and stir gently to coat with gravy. Serve immediately.