Preheat oven to 350 degrees F. Lightly grease a 2-quart rectangular baking dish; set aside. In a very large bowl, combine soup, cheese, milk, sour cream, onion, and pepper. Stir in potatoes. Spread mixture evenly in prepared baking dish.
Cover and bake for 45 minutes; stir potatoes. Sprinkle with cornflakes. Bake, uncovered, for 20 to 25 minutes more or until heated through and bubbly. Let stand for 10 minutes before serving.