Preheat oven to 350°F. Lightly grease a shallow 2-quart baking dish.
Combine corn, corn muffin mix, sour cream, butter, Mustard, egg and 2/3 cup Onions. Pour mixture into prepared baking dish
Bake 40 minutes until mixture is set. Top with remaining Onions and bake 5 minutes or until Onions are golden.