Grease one 9x5-inch loaf pan (or spray sides and bottom of loaf pan).
In the bowl of you mixer, cream together the butter and sugar until light and fluffy; add eggs and mix until blended. Stir in the mashed bananas and the sourdough starter. Stir in the vanilla extract or orange zest; set aside.
In another bowl, combine the flour, baking powder, baking soda, cinnamon, nutmeg, and salt.
Add the flour mixture, chopped nuts, and shredded zucchini squash to the prepared sourdough mixture, stirring just until blended.
Pour batter into prepared loaf pan and let the filled loaf pan aside for 15 to 20 minutes before baking. This is not your usual sourdough bread recipe where you let the dough rise until doubled. You only let it rest for approximately 20 minutes.
Preheat oven to 350 degrees F. Adjust oven rack to middle position.
Bake approximately 60 to 65 minutes (it took me 65 minutes to achieve the proper temperature) or until a toothpick inserted in the center comes out clean. Be sure and poke the toothpick already to the center of the loaf.A good check is to use an instant digital thermometer to test your bread. The internal temperature of the bread should be at 200 degrees F. when done.
Remove from oven and cool on a wire rack 10 minutes; remove from pan. Cool completely before slicing.
Storage: Breads that contain fruit, nuts, or butter may be stored for 2 or 3 days, well wrapped in plastic wrap or aluminum foil. May be frozen for longer storage. After thawing frozen breads, reheat them in a 400 degree F. oven for a few minutes before serving.
Makes one (1) large loaf.
* To ripen bananas, allow green or yellow bananas to ripen at room temperature until the skin is covered with brown spots.
** Make sure your sourdough starter is fresh and bubbly by adding equal amounts of water and flour to it the day before using. If you don't presently have a sourdough starter, either make your own sourdough starter or purchase Packaged Sourdough Starter Mix by mail-order.
Sourdough Banana Zucchini Bread Recipe: https://whatscookingamerica.net/Bread/SourdoughBananaZucchiniBread.htm