Preheat the oven to 350 degrees F (175 degrees C).
Combine hash browns, Cheddar cheese, condensed soup, sour cream, 1/2 cup melted butter, onion, salt, and pepper in a large bowl. Transfer mixture to a 3-quart casserole dish.
Saute cornflakes and remaining 1/4 cup melted butter in a saucepan over medium heat. Sprinkle the mixture over the top of the casserole.
Cover casserole and bake in the preheated oven for 40 minutes.