1. Preheat your oven to 400°F (200°C). Prepare a baking sheet by lining it with parchment paper.
2. In a large bowl, combine the ground beef, bread crumbs, egg, salt, pepper, and Worcestershire sauce. Mix until well combined, but be careful not to overwork the meat.
3. Shape the mixture into meatballs, about 1.5 inches in diameter, and place them on the prepared baking sheet.
4. Bake the meatballs for 15-20 minutes, or until they are browned and cooked through. While they bake, you’ll have time to prepare the onion mixture.
5. In a large skillet, heat the olive oil over medium heat. Add the onions and cook, stirring often, until they are soft and caramelized, about 15 minutes. This step requires patience, but trust me, it’s worth it!
6. Add the garlic to the onions and cook for another minute. Then pour in the beef broth, stirring to deglaze the pan and incorporate all those delicious browned bits.
7. Once the meatballs are done, add them to the skillet with the onions. Let them simmer in the sauce for an additional 5 minutes to soak up the flavors.
8. Sprinkle the shredded Gruyère cheese over the meatballs and cover the skillet. Let the cheese melt, creating a deliciously gooey topping.
9. Garnish with fresh thyme before serving to add a touch of freshness and color.

These meatballs remind me of the warmth and heart that went into every meal growing up, and I hope they bring the same to your table.