In a large bowl, combine the hash browns, soup, milk, cheese, 1/2 cup butter, onion, salt and pepper. Pour into a greased 5-qt. slow cooker. Cover and cook on low until potatoes are tender, 3-4 hours. , Just before serving, combine the cornflake crumbs and remaining butter in a pie plate. Bake at 350° until golden brown, 4-6 minutes. Stir the potatoes; sprinkle with crumb topping.