Preheat oven to 325 degrees F (165 degrees C). Grease 2 - 9x5 inch loaf pans.
In a large bowl, mash together the shortening and bananas, then stir in the starter, eggs, milk, sugar, and walnuts. Sift together the baking powder, baking soda, salt, and flour. Fold into the banana mixture. Divide batter evenly into the prepared pans.
Bake in preheated oven for 1 hour and 15 minutes, or until a toothpick inserted into the center of the loaf comes out clean. Cool loaf until it loosens evenly from the pan, about 10 minutes, before removing to a wire rack to cool completely.