Preheat the oven to 375°F (190°C).
In a large bowl, combine the chicken, rice, cream of mushroom soup, chicken broth, onion, mixed vegetables, garlic powder, salt, and pepper.
Spread the mixture evenly in a greased 9x13 inch baking dish.
Cover the dish with aluminum foil and bake for 35 minutes.
Remove the foil, sprinkle cheddar cheese on top, and return to the oven for an additional 10-15 minutes.
Check that the rice is cooked and the chicken is no longer pink.
Let the casserole cool for about 5 minutes before serving.