Prepare the meatballs: In a large bowl, mix ground beef, breadcrumbs, egg, Worcestershire sauce, garlic powder, onion powder, salt, and black pepper. Form into 1-inch meatballs.
Brown the meatballs (optional): Heat olive oil in a skillet over medium-high heat and lightly brown the meatballs for extra flavor.
Assemble in the slow cooker: Place sliced onions and mushrooms in the slow cooker, then add the meatballs on top.
Make the sauce: In a bowl, whisk together beef broth, Worcestershire sauce, ketchup, and Dijon mustard. Pour over the meatballs.
Slow cook: Cover and cook on LOW for 6 hours or HIGH for 4 hours until the meatballs are tender.
Thicken the gravy: In the last 30 minutes, stir in the cornstarch slurry and let the sauce thicken.
Serve hot: Serve the meatballs and gravy over mashed potatoes, rice, or egg noodles.