Creating the French Onion Chicken Orzo Casserole is quite easy if you follow these simple steps:

1. Preheat Oven: Preheat your oven to 350°F (180°C).
2. Sauté Onions: In a large skillet over medium heat, add the olive oil. Once heated, add the sliced onions, cooking until caramelized, which takes about 15 minutes. Stir occasionally to prevent burning.
3. Season Chicken: While onions are cooking, season the chicken pieces with salt, pepper, and thyme.
4. Cook Chicken: In the same skillet, add the seasoned chicken to the caramelized onions. Cook until the chicken is no longer pink, about 5-7 minutes.
5. Add Orzo: Stir in the orzo pasta and mix well to combine all ingredients. Allow it to absorb the flavors for a minute.
6. Incorporate Broth and Cream: Pour in the chicken broth and heavy cream, stirring until everything is evenly mixed. Bring to a low simmer.
7. Transfer to Casserole Dish: Once combined, pour the mixture into a greased casserole dish.
8. Top with Cheese: Sprinkle the grated Gruyère and shredded mozzarella cheese evenly over the top.
9. Bake: Place the casserole in the preheated oven and bake for 25-30 minutes or until the cheese is melted and bubbly.
10. Garnish and Serve: Remove from the oven and let it sit for a few minutes before garnishing with extra thyme if desired.

This step-by-step guide will lead you through creating this fantastic casserole, ensuring a delicious outcome.