Preheat broiler. Put chiles on a baking sheet and broil 4 in. from heat until blackened all over, turning as needed, about 15 minutes. Let chiles cool, then remove and discard stems, seeds, and skins. Coarsely chop chiles and set aside. Preheat oven to 400°.
Stir together flour, cornmeal, sugar, baking powder, and salt in a large bowl. In another bowl, beat eggs, buttermilk, and butter to blend. Pour egg mixture into flour mixture; add chiles, corn, and onions; and stir just until blended. Spread in a buttered 9- by 13-in. baking dish.
Bake until a toothpick inserted in center comes out clean, 25 to 30 minutes. Let cool at least 20 minutes. Serve warm or cool, cut into pieces.
*Buy in a grocery store or natural-food store.
Make ahead: Up to 1 day. Wrap in foil and reheat in a 350° oven about 10 minutes (set bread on top of something if oven is full).