Preheat oven to 375°F (190°C).
In a large skillet over medium heat, melt butter and add sliced onions. Sauté for 15-20 minutes, stirring occasionally, until caramelized and deep golden brown.
Add ground beef to the skillet and cook until fully browned, breaking it into small crumbles. Drain excess grease from the pan.
Stir in minced garlic, dried thyme, salt, and pepper; cook for 1 minute until fragrant.
Add the uncooked white rice, beef broth, and Worcestershire sauce to the beef mixture. Stir to combine and bring to a simmer.
Transfer the contents to a 9x13 inch baking dish and spread evenly. Cover tightly with aluminum foil.
Bake for 30 minutes until the rice is tender and the liquid is mostly absorbed.
Remove from oven and discard foil. Sprinkle the shredded Gruyère cheese evenly over the top.
Return to the oven and bake uncovered for 5-10 minutes, or until the cheese is melted and bubbly.
Garnish with fresh parsley and allow to rest for 5 minutes before serving.