Preheat oven to 425 degrees. Coat a 13”x9” baking dish with nonstick spray.
In a microwave-safe bowl, add the onions and butter. Cook in the microwave on high for about 4 minutes until soft. Set aside.
In a medium bowl, mix together the egg whites, egg and milk. Stir in the cornbread mix and cream-style corn. Spread cornbread batter into the prepared dish.
Spoon the cooked onions and liquid left in the bowl evenly over the top.
Using a rubber spatula carefully spread the sour cream over the onions. It’s not necessary to completely cover the onions. The sour cream will create little pillows.
Sprinkle the cheese evenly over the top of the sour cream.
Bake for about 40-45 minutes until golden brown.
Let stand for 10 minutes before cutting into 16 squares. This casserole may be made ahead of time, refrigerated and reheated. It also freezes great.