1. Preheat the oven to 375°F.

2. In a large oven-safe skillet or Dutch oven, heat the olive oil over medium-low heat. Add the sliced onions and cook, stirring occasionally, for 20-25 minutes until the onions are caramelized and golden brown.

3. Add the minced garlic to the skillet and cook for an additional 2 minutes, until fragrant.

4. Add the chicken pieces to the skillet and cook for 5-7 minutes, stirring occasionally, until the chicken is lightly browned.

5. Pour in the chicken broth and stir in the orzo, thyme, salt, and black pepper. Bring the mixture to a boil.

6. Once boiling, cover the skillet and transfer it to the preheated oven. Bake for 15-20 minutes, or until the orzo is tender and the liquid is mostly absorbed.

7. Remove the skillet from the oven and sprinkle the shredded Gruyère or Swiss cheese over the top.

8. Return the skillet to the oven and bake for an additional 5-10 minutes, or until the cheese is melted and bubbly.

9. Serve the French Onion Chicken Orzo Casserole hot, garnished with fresh thyme if desired.