In a large bowl, combine the cornmeal, flour, baking powder, sugar, salt and baking soda. Melt the butter in a medium skillet over medium heat. Cook the onions until soft, about 5 minutes. Add chipotle paste and set aside.

Grease a cast iron skillet (6 inches in diameter and at least 1inch deep), with 1 teaspoon of the shortening and heat skillet in a 450-degree oven for 10 minutes. Add the remaining shortening to the cornmeal mixture and blend until it resembles coarse crumbs. Add the onion mixture and gradually stir in the buttermilk so that batter that barely holds its shape. Stir until just mixed.

Pour batter into the hot skillet and bake it in the middle of a 450 degrees oven for 10 to 12 minutes. Cool for 5 minutes in the pan. Place a rack over the skillet, turn skillet over and invert bread onto rack.

Chile Pastes
This recipe is for guajillo paste but the method will work for most any chile including chipotle chiles.