Preheat the oven to 350°F (175°C).
In a large bowl, combine the shredded chicken, sliced mushrooms, cooked rice, and 1/2 cup of the shredded cheddar cheese.
In a separate bowl, whisk together the cream of mushroom soup, milk, garlic powder, salt, and black pepper.
Pour the soup mixture over the chicken and mushroom mixture, stirring until well combined.
Transfer the mixture to a greased baking dish and spread it out evenly.
Sprinkle the remaining cheddar cheese and breadcrumbs on top.
Bake in the preheated oven for 25-30 minutes, or until the top is golden and bubbly.
Let it cool for a few minutes before serving.