Preheat the oven to 400°F (200°C) and line a baking sheet with parchment paper.
In a skillet, heat the olive oil over medium heat. Add the chopped onion and cook until caramelized, about 8-10 minutes, stirring occasionally to prevent burning.
In a mixing bowl, combine the ground beef, breadcrumbs, egg, minced garlic, salt, pepper, Worcestershire sauce, and half of the caramelized onions. Mix until well combined.
Form the mixture into meatballs, about 1 inch in diameter, and place them on the prepared baking sheet.
Bake the meatballs in the preheated oven for 20 minutes or until they reach an internal temperature of 160°F (70°C).
While the meatballs are baking, prepare the broth in a saucepan by heating the beef broth and the remaining caramelized onions over low heat. Keep warm.
Once the meatballs are finished baking, add them to the saucepan with the broth and stir gently to combine.
Serve the meatballs warm, topped with shredded Gruyère cheese and garnished with fresh parsley.