Preheat the oven to 350 degrees F. Grease a 9- by 13- by 2-inch baking dish.
Place the bread cubes on a large baking sheet and toast in the oven until lightly brown, 30 minutes, turning once after 15 minutes. Set aside to cool. Raise the oven temperature to 450 degrees F.
In a medium skillet, saute the onions in the turkey fat until translucent and softened but not browned, about 5 minutes. In a very large bowl, crumble the Buttermilk Cornbread, toasted bread cubes and cracker crumbs. Add the onions and eggs and toss with a fork until mixed. Add 3 cups of the broth and mix it well, adding more as needed to make a very moist but not soupy dressing. Season with salt and pepper. Bake in the prepared pan until slightly browned, about 15 minutes.
Preheat the oven to 450 degrees F.
Pour 2 tablespoons of the oil into a well-seasoned 9-inch cast-iron skillet and place over medium-high heat. Put the cornmeal in a large mixing bowl. Make a well in the cornmeal and add the remaining 2 tablespoons oil. With a fork, stir in enough of the buttermilk to make a batter that is thick but can be easily poured into the hot skillet. You may not need all 3 cups.
Carefully pour the batter into the skillet. The oil will come up around the edges. Use the back of a spoon to smooth this over the top of the batter. Continue to heat on the stovetop for 1 minute, and then transfer the skillet to the oven and bake the cornbread until browned on top, about 20 minutes. Immediately turn the cornbread out onto a cooling rack to keep the crust crisp.
Recipe adapted from Georgia Cooking in an Oklahoma Kitchen by Trisha Yearwood (c) Clarkson Potter 2008