Combine first 4 ingredients and 1 tablespoon butter in a large bowl; toss well.
Combine soup and cheese in a medium microwave-safe bowl. Microwave at HIGH 6 minutes or until cheese melts, stirring every 2 minutes. Stir in sour cream. Pour cheese mixture over potato mixture, and stir well. Spread mixture evenly in a 13 x 9-inch baking dish coated with cooking spray.
Combine cornflakes and remaining 1 tablespoon butter; sprinkle evenly over top of potato mixture. Bake at 350° for 1 hour.