Preheat the oven to 350°F (175°C).
In a medium saucepan, bring 4 cups of water to a boil. Add the uncooked rice, reduce heat to low, cover, and simmer for 15-20 minutes until the rice is tender and water is absorbed. Remove from heat.
In a large mixing bowl, combine Campbell’s Cream of Mushroom Soup, Campbell’s Cream of Chicken Soup, sour cream, milk, garlic powder, black pepper, and salt. Stir until smooth.
Add the cooked rice, cooked chicken, chopped onion, and sliced mushrooms (if using) to the soup mixture. Stir gently to combine all ingredients evenly.
Lightly grease a 9×13 inch casserole dish with butter or non-stick spray.
Pour the mixture into the prepared casserole dish and spread evenly.
Sprinkle the shredded cheddar cheese evenly over the top of the casserole.
Drizzle the melted butter over the cheese layer to help it brown.
Cover the casserole dish with aluminum foil and bake in the preheated oven for 40 minutes.
Remove the foil and bake for an additional 15-20 minutes, or until the cheese is bubbly and golden brown.
Remove from oven and let the casserole rest for 5 minutes before serving.