Preheat oven to 425°F.
Grease 9x 13-inch baking dish.
Heat butter in skillet over medium-high heat and saute onions until golden.
Combine eggs and buttermilk in large bowl and mix until smooth.
Stir in corn and muffin mix, blending well.
Turn into baking dish and spoon onion'evenly over batter.
Spread sour cream over onion and top with cheese.
Bake until golden, about 30 minutes.
Let stand 10 minutes before cutting.