Pre-heat oven to 350 degrees and spray an 8 ½ x 4 ½ inch bread pan with cooking spray.
In a medium bowl, whisk together whole wheat pastry flour, flaxseed meal, cinnamon, baking soda, nutmeg and salt.
Cream butter and sugar in a stand mixer over medium-high speed until smooth, about 2-3 minutes.
Reduce stand mixer speed to low and slowly add the eggs one at a time followed by the vanilla, mashed bananas, and yogurt. Use a rubber spatula to scrape down sides and increase speed to medium until ingredients are blended.
With the mixer still running, slowly add the dry ingredients until they are all added. Scrape down the sides one last time ensuring that the wet and dry ingredients are fully incorporated.
Remove bowl from the stand mixer and stir in the walnuts.
Pour the batter into the prepared bread pan and bake for one hour or until toothpick comes out smooth. Cool completely before slicing and serving.