Preheat the oven to 350 degrees F. Coat a 9- by 5-inch loaf pan with cooking spray.
In the bowl of a stand mixer (or in a large bowl, with a hand mixer), combine the mashed bananas, Greek yogurt, peanut butter and canola oil. Mix on medium speed until combined.
Add the granulated and brown sugars, and mix again until combined.
Add the eggs one at a time, beating between each addition.
In a medium-sized bowl, whisk together the flour, ground flax seed, baking soda, salt, cinnamon and nutmeg.
With the mixer on low, gradually add the flour mixture to the banana mixture. Beat until just combined.
Pour the batter into the prepared loaf pan and spread evenly with a rubber spatula. Tap the pan sharply two times on the counter to get rid of any air bubbles.
Bake until a toothpick inserted in the center comes out clean, approximately 1 hour. Allow the bread to cool in the pan for 15 minutes. Remove from the pan and cool completely on a wire rack. Serve.