In a large mixing bowl, combine ground beef, French onion soup, breadcrumbs, egg, garlic powder, onion powder, salt, and pepper. Mix until just combined.
Roll the mixture into small meatballs, about 1 to 1.5 inches in diameter.
Place the meatballs in the Crock Pot snugly.
Pour any remaining French onion soup over the meatballs.
Cover and cook on low for 6-8 hours or on high for 3-4 hours.
If using, sprinkle shredded mozzarella cheese over the meatballs about 10 minutes before serving and cover until melted.
Notes
Store leftovers in an airtight container in the fridge for 3-4 days or freeze for up to 3 months.