Preheat the oven to 350°F (175°C).
In a large mixing bowl, combine the cooked chicken, Rice-A-Roni (dry), sour cream, cream of chicken soup, garlic powder, onion powder, frozen mixed vegetables, salt, and pepper.
Pour the mixture into a greased 9x13 inch baking dish, spreading it out evenly.
Cover the dish with aluminum foil and bake for 30 minutes.
Remove the foil, sprinkle shredded cheddar cheese on top, and return it to the oven uncovered for an additional 10-15 minutes.
Once the cheese is bubbly and golden, remove the casserole from the oven and let it cool for a few minutes before serving.