Make the Meatballs:
In a large bowl, mix ground beef, breadcrumbs, egg, Worcestershire sauce, garlic powder, onion powder, salt, and pepper. Shape into 1-inch balls.
Brown the Meatballs (Optional):
Heat a skillet with a little oil and brown the meatballs on all sides. This adds flavor and helps them hold their shape.
Mix the Gravy:
In a medium bowl, whisk together the cream of mushroom soup, beef broth, ketchup, and Worcestershire sauce until smooth.
Assemble the Crockpot:
Add sliced onions and mushrooms to the bottom of your slow cooker. Place the meatballs on top. Pour the gravy mixture over everything.
Cook:
Cover and cook on Low for 5–6 hours or High for 3 hours, until meatballs are tender and cooked through.
Thicken (Optional):
If you prefer a thicker sauce, mix cornstarch with a tablespoon of water and stir it in during the last 15 minutes.
Serve:
Spoon meatballs and gravy over mashed potatoes, rice, or noodles. Garnish with parsley and serve warm.