1. Preheat your oven to 400°F (200°C). Line a baking sheet with parchment paper.
2. In a large mixing bowl, combine the ground beef, breadcrumbs, egg, Parmesan cheese, Worcestershire sauce, salt, and pepper. Mix until all ingredients are well incorporated.
3. Shape the mixture into 1.5-inch meatballs and arrange them on the prepared baking sheet.
4. Bake the meatballs in the preheated oven for 15-20 minutes, or until they’re browned and cooked through.
5. While the meatballs are baking, heat olive oil in a large skillet over medium heat. Add the sliced onions and cook, stirring occasionally, until they’re caramelized, about 20 minutes. Adjust the heat as necessary to prevent burning.
6. Add butter, beef broth, and thyme to the skillet with the onions. Bring the mixture to a simmer and let it cook for an additional 5 minutes.
7. Once the meatballs are cooked, transfer them into the skillet with the onion mixture. Sprinkle the shredded Gruyère cheese over the top and cover the skillet, allowing the cheese to melt.
8. Serve the meatballs hot, garnished with fresh thyme or parsley if desired.

My mother always said, “Patience is key to perfect caramelized onions,” and she was right. Take your time with this step for the best flavor.