Transfer 2 tablespoons of bacon drippings in skillet to a small bowl; set aside. Drain and discard all but 2 tablespoons of the remaining drippings from skillet. Cook onion in hot drippings in skillet over medium heat about 6 minutes or until dark brown. Remove from heat; set aside.
Brush the reserved bacon drippings over the bottom and sides of a 2-quart square baking dish. Sprinkle bottom and sides of dish generously with cornmeal. Set aside.
In a large mixing bowl combine warm milk and yeast; let stand until mixture is foamy. Add butter, egg, salt, and 1 cup of the flour. Beat with an electric mixer on medium speed for 2 minutes, scraping sides of bowl occasionally. Using a wooden spoon, stir in the remaining 2 cups flour, the mashed potatoes, bacon, and onion until a soft, sticky dough forms.