Preheat oven to 375°F (190°C) and lightly grease a 9x13 inch baking dish.
In a large skillet over medium heat, melt the butter. Add the sliced onions and cook, stirring frequently, for 15-20 minutes until caramelized and golden brown.
Add the minced garlic and dried thyme to the skillet; cook for 1 minute until fragrant.
In the prepared baking dish, combine the uncooked rice, beef broth, Worcestershire sauce, salt, and pepper.
Stir in the caramelized onion mixture and the raw chicken pieces, ensuring they are distributed evenly throughout the rice and liquid.
Cover the dish tightly with aluminum foil and bake for 35 minutes.
Remove the foil, sprinkle the shredded Gruyere cheese evenly over the top, and bake uncovered for another 10 minutes until the rice is tender and the cheese is melted and bubbly.
Remove from oven and allow to rest for 5 minutes to let the rice absorb any remaining moisture before serving.