In a small bowl, combine the ground flax seed and water. Stir well and let it sit for about 5 minutes until it thickens.
In a large mixing bowl, mash the ripe bananas until smooth.
Add the thickened flax egg, avocado oil, vanilla extract, and coconut sugar to the mashed bananas. Mix until well combined.
Gently fold in the sourdough starter to the banana mixture.
In another bowl, whisk together the gluten-free all-purpose flour, baking soda, sea salt, and ground cinnamon.
Gradually add the dry ingredients to the wet mixture, stirring gently until just combined.
Fold in the chopped walnuts, reserving a few for topping if desired.
Preheat your oven to 350°F (175°C). Grease your loaf pan and pour the batter into it. Bake for 50-60 minutes or until a toothpick comes out clean.
Allow the bread to cool in the pan for 10 minutes before transferring it to a wire rack to cool completely.
