1. Preheat oven to 350°F. Mix the bananas, yogurt, eggs, oil, and vanilla together in the bowl of a stand mixer until combined. Add the sugar and continue to mix until combined.
2. Whisk the flours, flaxseed, baking soda, cinnamon, and salt together in a medium bowl. Add the dry ingredients to the wet ingredients in the mixer and beat until just combined (do not over-mix). Stir the nuts into the batter.
3. Pour the batter into a 9 x 5-inch loaf pan coated with cooking spray. Bake in the oven for 50 to 55 minutes, or until a toothpick inserted into the center comes out clean. Remove from oven and cool for 10 minutes in the pan on a wire rack. Remove the bread and cool completely on wire rack.