Preheat your oven to 350°F (175°C).
In a large mixing bowl, combine the cooked chicken, Rice-A-Roni, cream of chicken soup, and milk.
Add the mixed vegetables, garlic powder, onion powder, salt, and pepper to the mixture.
Transfer the mixture to a greased 9x13-inch baking dish.
Sprinkle the shredded cheese evenly over the top.
Cover the dish with aluminum foil and bake for 30 minutes.
Remove the foil and bake for an additional 15 minutes, or until the cheese is bubbly and golden.
Let the casserole cool for a few minutes before serving.