Preheat the oven to 350 degrees Fahrenheit. Spray a 9x13 inch baking dish with cooking spray and set aside.
In a large bowl, combine the cream of chicken soup, sour cream, melted butter, diced onion, salt, pepper, and garlic powder and mix until smooth.
Add the thawed hashbrowns and 2 cups of cheese and mix to coat everything well.
Pour into the prepared baking dish and spread evenly with a spatula.
Sprinkle the remaining 1 cup of cheese on top and bake for 45-50 minutes, until bubbly and golden at the edges.
