Preheat oven to 350°F (175°C) and grease a 9x13 inch baking dish.
In a large skillet, melt butter over medium heat and sauté onions until deeply caramelized, approximately 20 minutes.
Add ground beef to the skillet and cook until browned; drain excess fat.
Stir in garlic, thyme, salt, and pepper, cooking for 1 minute until fragrant.
In the prepared baking dish, combine the beef mixture, uncooked rice, beef broth, condensed French onion soup, and Worcestershire sauce.
Cover the dish tightly with aluminum foil and bake for 45 minutes, or until rice is tender and liquid is absorbed.
Remove foil, sprinkle evenly with shredded Gruyère cheese, and bake for an additional 5-10 minutes until cheese is melted and bubbly.
Let stand for 5 minutes before serving.