Preheat oven to 350°F (175°C) and grease a 9x13-inch baking dish with butter or non-stick spray.
In a large mixing bowl, combine the thawed hash browns, sour cream, cream of chicken soup, shredded cheddar cheese, 1/2 cup melted butter, finely diced onion, salt, and black pepper.
Stir the mixture thoroughly with a large spatula until all ingredients are evenly distributed and the potatoes are fully coated.
Transfer the mixture to the prepared baking dish and spread into an even layer.
In a separate small bowl, toss the crushed cornflakes with 2 tablespoons of melted butter until the flakes are evenly coated.
Sprinkle the buttered cornflake crumbs uniformly over the top of the hash brown mixture.
Bake in the center of the oven for 45 to 50 minutes until the casserole is bubbling around the edges and the topping is deeply golden brown.
Remove from the oven and allow the casserole to rest for 5 to 10 minutes before serving to set the creamy texture.