Preheat the oven to 400°F.
In a large bowl, combine the ground beef, breadcrumbs, Parmesan cheese, chopped onion, parsley, egg, garlic, salt, pepper, thyme, and Worcestershire sauce. Mix until just combined.
Shape the mixture into meatballs, about 1 to 1.5 inches in diameter, and place them on a baking sheet lined with parchment paper.
Bake for 20-25 minutes or until cooked through and browned.
While the meatballs are baking, heat the beef broth in a saucepan over medium heat. Stir in the caramelized onions and let simmer for about 5 minutes.
Once the meatballs are done, remove them from the oven and place them in the saucepan with the broth and onions. Gently stir to coat the meatballs in the sauce.
Sprinkle the shredded Gruyère cheese over the meatballs and cover the saucepan. Allow the cheese to melt for about 3-5 minutes.
Serve the meatballs warm, garnished with additional parsley if desired.