In a large mixing bowl, combine the ground beef, breadcrumbs, chopped onion, egg, Worcestershire sauce, garlic powder, salt, and black pepper. Mix until well blended.
Form the mixture into meatballs, about 1 to 1.5 inches in diameter.
Optional: Heat a skillet over medium heat and lightly brown the meatballs on all sides for additional flavor. Remove them from the skillet and place them directly into the slow cooker.
In a separate bowl, whisk together the cream of mushroom soup and beef broth until smooth. Pour this mixture over the meatballs in the slow cooker. Add the sliced mushrooms on top.
Cover and cook on low for 6 hours or on high for 3 hours until the meatballs are cooked through and tender.
Optional: If you prefer a thicker gravy, mix the cornstarch and water in a separate bowl to create a slurry. Add this to the slow cooker during the last 30 minutes of cooking, stirring gently to combine.
Serve the Salisbury steak meatballs over mashed potatoes or noodles and enjoy!