Preheat oven to 375°. Lightly grease a 13"x9"x3" baking pan and set aside. In a 12" skillet, heat the oil over moderate heat. Add the onion and sauté, stirring frequently, for 5 minutes or until softened. Add the green, red and jalapeño peppers and sauté, stirring frequently, 3 minutes longer. Stir in the corn and cook 1 minute more. Remove from the heat and set aside.
In bowl, stir together the flour, cornmeal, sugar, baking powder, baking soda and salt until well combined. Stir in the pepper mixture. With a mixing spoon, make a well in the center. In a small bowl, stir together the buttermilk, oil, egg and egg whites until well combined. Pour this mixture into the well and stir until just combined.
Scrape the batter into the prepared pan. Bake for 25 minutes or until a toothpick inserted in the center comes out clean. Serve hot or warm. Makes 12 servings.