Mix together the sourdough starter, flour, milk, maple syrup, butter, cinnamon, and pumpkin pie spice. Cover and let sit for 8 hours.
When ready to bake the bread, preheat your oven to 325 degrees and line your loaf pan with parchment or spray with oil.
In a separate bowl, combine the bananas, pumpkin puree, eggs, vanilla, baking soda, baking powder, salt, brown sugar, and walnuts(if using).
Add your mixture into the bowl with the sourdough mixture and mix until everything is fully combined.
Pour your batter into the prepared loaf pan.
Sprinkle walnuts and 1-2 tablespoons of brown sugar on top of your batter.
Bake for 75-90 minutes.
Let cool before removing from loaf pan or slicing.