1. In a large skillet, melt butter over medium heat. Add sliced onions and sauté until caramelized, about 20–25 minutes. Stir frequently to avoid burning. Set aside.

2. In the same skillet, cook ground beef until browned. Drain excess fat, then add garlic, Worcestershire sauce, salt, and pepper. Cook 1 more minute. Remove from heat.

3. In a medium saucepan, bring beef broth to a boil. Stir in rice, reduce heat to low, cover, and simmer until rice is cooked and broth absorbed, about 15–18 minutes.

4. Preheat oven to 375°F (190°C). In a large bowl, mix cooked rice, beef mixture, caramelized onions, and sour cream (if using).

5. Spread mixture evenly in a greased 9×13-inch baking dish. Top with mozzarella and Gruyère cheese.

6. Cover with foil and bake for 20 minutes. Uncover and bake an additional 10 minutes or until cheese is golden and bubbly. Rest 5–10 minutes before serving.