In a mixing bowl, combine the ground beef, breadcrumbs, chopped onion, egg, garlic powder, Worcestershire sauce, salt, pepper, and Italian seasoning. Mix until well combined.
Form mixture into meatballs about 1-1½ inches in diameter.
In a large skillet, heat a little oil over medium heat. Brown the meatballs in batches for 2-3 minutes until they are nicely seared on all sides. Remove and set aside.
In the crock pot, whisk together the beef broth and the French onion soup mix until dissolved.
Add the meatballs to the crock pot, ensuring they are submerged in the broth mixture.
Cover and cook on low for 6 hours or on high for about 3 hours, until the meatballs are cooked through.
For the last 30 minutes of cooking, sprinkle the shredded mozzarella cheese over the meatballs. Cover to melt the cheese.
Serve the meatballs warm, drizzled with some of the French onion broth.