Grease a 9x13-inch baking dish with cooking spray.
Combine the hashbrowns, salt, garlic powder, and black pepper in a large bowl. Gently stir with a silicone spatula to evenly incorporate the spices.
Add the condensed cream of chicken soup, butter, and onion, and stir to combine. Fold in the cheese until evenly distributed.
Lightly spoon the mixture into the prepared baking dish, being careful not to press down on the hashbrowns.
Bake the casserole for 30 to 35 minutes, until the top is golden brown and crisp. Serve warm.
