In a large mixing bowl, combine the ground beef, breadcrumbs, Parmesan cheese, chopped onion, minced garlic, egg, Worcestershire sauce, salt, and black pepper. Mix until well combined.
Shape the mixture into 12 to 14 meatballs, about the size of a golf ball.
Optional: Heat olive oil in a skillet over medium heat. Brown the meatballs on all sides for about 3-4 minutes. This step adds extra flavor but can be skipped if you'd like to save time.
Place the meatballs in the slow cooker.
In a separate bowl, whisk together the beef broth and onion soup mix until well blended. Pour this mixture over the meatballs in the slow cooker.
Add the sliced mushrooms on top of the meatballs.
Cover the slow cooker and cook on low for 6 hours or high for 3 hours, until the meatballs are cooked through.
Once cooked, serve the meatballs with the gravy over mashed potatoes or egg noodles, and garnish with chopped parsley.