Set your oven to preheat at 350°F (175°C). Lightly coat a 9x13-inch rectangular baking dish with butter or a nonstick spray.
In a spacious mixing bowl, combine the thawed shredded potatoes, cream of chicken soup, sour cream, grated cheddar, diced onion, melted butter, salt, pepper, and garlic powder. Stir the mixture thoroughly until all components are evenly incorporated.
Transfer the combined mixture into the prepared baking dish, spreading it into an even layer.
In a separate smaller bowl, mix together the crushed cornflakes and 1/4 cup of melted butter until well coated. Evenly distribute this topping across the surface of the casserole.
Bake the dish uncovered for 45 to 50 minutes, or until the surface is a deep golden hue and the interior is visibly bubbling.
Allow the casserole to rest for 5 to 10 minutes prior to slicing and serving.