Melt butter in frying pan.
Add onion and saute until golden brown.
Pour off butter in 9x9x2 cake pan.
Pour muffin mix into bowl.
In separate bowl, blend egg, milk and corn.
Pour into muffin mix and mix well.
Spread in baking pan.
Top with onions, then sour cream and cheese.
Bake 35 minutes at 425 degrees or until golden brown.