Preheat oven to 350°F. Lightly grease a 9- x 13-inch baking dish.
Combine the sour cream, cream of chicken soup, salt, pepper, and garlic powder in a large mixing bowl, stirring with a whisk.
Fold in the thawed hash brown potatoes, Cheddar cheese, and onion.
Pour the mixture into prepared dish and spread into an even layer.
Combine the crushed cornflakes and melted butter in a small mixing bowl; stir to coat.
Sprinkle the butter-coated cornflakes evenly over the potato mixture.
Bake in preheated oven for 35-40 minutes, or until the top is crispy and golden brown.
