Preheat oven to 350 degrees F. Lightly spray a 9" x 5" loaf pan with cooking oil

In a small bowl, stir together the ground flax seed with the water. Set aside for 10-15 minutes, stirring occasionally, until thickened to the consistency of a beaten egg.

In a large bowl, combine the flours, baking soda, salt, and cinnamon.

In a medium bowl, whisk together the starter, mashed banana, flax seed mixture, vanilla, oil, and coconut sugar. Add to the dry ingredients and stir until just combined. Gently fold in the walnuts.

Pour the batter into the prepared loaf pan. If desired, sprinkle the top generously with extra walnuts. Bake for 50-60 minutes, until bread tests clean.

Allow bread to cool before removing from the pan, then set on a rack to cool completely.