Preheat oven to 350 degrees. Grease a 9x5-inch loaf pan; set aside. 2. In a small bowl, combine the ground flaxseed with the water. Set aside for 10-15 minutes, or until thickened and the consistency of an egg.
In a large bowl, combine the flours, baking soda, and salt.
In a medium bowl, whisk together the starter, mashed banana, flaxseed mixture (your "flax egg"), vanilla, oil, and brown sugar. Add to the dry ingredients and stir until just combined. Gently fold in the walnuts or pecans.
Pour the batter into the prepared loaf pan. Put a thin banana slice on top of each and sprinkled brown sugar over the banana slice. Bake for 50-60 minutes, until a toothpick inserted into the center comes out clean. Allow to cool before removing from the pan, then set on a rack to cool completely.
For muffins, pour batter into muffin tins lightly sprayed with cooking spray. Put a thin slice of banana in the center of each muffin and sprinkle brown sugar over the top. Bake at 350℉ for about 20 minutes or until a toothpick inserted into the center comes out clean.