Making these meatballs is straightforward and rewarding. Here’s how you can bring this dish to life:

1. Preheat your oven to 375°F.
2. In a large bowl, combine ground beef, breadcrumbs, egg, salt, and pepper. Mix until just combined, being careful not to overwork the meat.
3. Form the mixture into meatballs, about 1 1/2 inches in diameter, and place them on a baking sheet lined with parchment paper.
4. In a large skillet, heat olive oil over medium heat. Add the sliced onions and cook, stirring occasionally, until they are caramelized and golden brown, about 15-20 minutes.
5. Add garlic and thyme to the onions and cook for another minute until fragrant.
6. Pour in the beef broth and Worcestershire sauce, scraping up any browned bits from the bottom of the skillet. Let the mixture simmer for 5 minutes.
7. Carefully add the meatballs to the skillet, spooning some of the onion mixture over the top. Sprinkle shredded Gruyère cheese over the meatballs.
8. Transfer the skillet to the preheated oven and bake for 15-20 minutes, until the meatballs are cooked through and the cheese is bubbly and golden.
9. Remove from the oven and let cool slightly before garnishing with fresh parsley, if desired.

From my experience, making sure the onions are perfectly caramelized is key to achieving the rich flavor that makes this dish stand out. Don’t rush this step; it’s worth the time!