Preheat oven to 350°F (175°C). Lightly grease a 9x13-inch baking dish.
In a large bowl, combine the soup, sour cream, 1/2 cup melted butter, onion, cheese, salt, and pepper.
Add the thawed hash browns and stir until evenly coated.
Spread the potato mixture evenly into the prepared baking dish.
In a small bowl, mix the crushed cornflakes with 1/4 cup melted butter.
Sprinkle the cornflake mixture evenly over the casserole.
Bake for 45 minutes, until golden and bubbly.