In a large bowl, combine the thawed hash browns, melted butter, cream of chicken soup, sour cream, 1 cup of cheddar cheese, chopped onion, salt, and pepper.
Spread the mixture evenly in a 9x13 inch baking dish.
In a small bowl, combine the crushed cornflakes with 0.25 cup melted butter. Sprinkle this mixture over the potatoes.
Top with the remaining 0.5 cup of cheddar cheese.
Bake in the preheated oven for 45 minutes, until the top is golden and the casserole is bubbly.
Notes
Let the casserole sit for a few minutes before serving to help it set up.