Preheat oven to 350 degrees F. Line a muffin tin with liners and set aside.
In a large bowl, make the flax egg by combining the ground flaxseed and water. Set aside for 5 minutes to thicken.
In a medium bowl, add the flour, baking soda, cinnamon, and salt. Whisk to combine.
In the large bowl with the flax egg, add the bananas and mash with a masher or fork. Add the zucchini, coconut sugar, oil, vanilla, and almond milk. Whisk until well combined. Add the starter and whisk until combined.
Add the dry ingredients into the bowl with the wet. Using a spatula, mix until just combined. Careful not to over mix. Fold in 3/4 cup of walnuts.
Fill muffin cups about 3/4 full. Sprinkle the last 1/4 cup of walnuts over the top. Bake for 20 minutes or until a toothpick comes out clean. Let it cool in the pan for 5 minutes. Carefully remove muffins from the pan and transfer to a wire rack to cool completely. Enjoy!