Zucchini Chicken and Rice casserole

So I have been looking everywhere to find recipes to help use up the many, many zucchini that we have. I was definitley pleasantly suprised with this one.

4 tbsp. vegetable oil, divided
2 cups onions-chopped
1 garlic clove-minced
3 cups zucchini-diced
2 cups mushrooms-sliced
3 cups chicken-cooked & diced
1 cup uncooked brown or white rice
2 1/2 cups chicken broth or water
1 1/2 tsp. fresh thyme (or 1/2 tsp. dried)
1/2 tsp. rosemary
1/2 cup grated Parmesan cheese
salt and pepper
2 cups chopped tomatoes
1/4 tsp. chili powder

Heat 3 tbsp. of the oil in a large saute pan, and saute 1 cup of the onions with the garlic for 2 minutes. Add the zucchini and mushrooms, and saute for 3 minutes more. Combine the vegetables with the chicken, rice, broth, and spices, and the cheese. Mix and season to taste with salt & pepper. Place this mixture in a greased 9x13 baking dish. Heat the remaining 1 tbsp. oil, and saute the remaining cup of onions for 3 minutes. Add the tomatoes and chili powder. Season to taste with salt and pepper. Spoon the topping over the rice and chicken mixture. Bake at 350 degrees for 1 hour. Serve hot. Casserole is quick to prepare and can be made up in advance and held until ready to cook in oven.