Preheat your oven to 350°F (175°C).
In a large mixing bowl, combine the shredded chicken, sliced mushrooms, cream of chicken soup, sour cream, garlic powder, onion powder, salt, and pepper.
Transfer the mixture to a greased 9x13 inch baking dish.
Sprinkle the shredded cheddar cheese evenly over the top of the mixture.
If desired, sprinkle breadcrumbs on top of the cheese.
Cover the baking dish with aluminum foil and bake for 25 minutes.
Remove the foil and bake for another 15 minutes, or until the cheese is bubbly and golden brown.
Let the casserole cool for a few minutes before serving.