Preheat oven to 425°.
Combine cornmeal mix and sugar in a large bowl; set aside. Combine 3 tablespoons butter, onion, and next 6 ingredients (onion through egg whites) in a medium bowl; stir well with a whisk. Add to cornmeal mixture, stirring just until moist. Pour batter into a 13 x 9-inch baking pan coated with cooking spray. Bake at 425° for 28 minutes or until a wooden pick inserted in center comes out clean. Brush with 2 tablespoons butter.