Preheat Oven: Start by preheating your oven to 350°F (175°C). Lightly coat a 9x5-inch loaf pan with cooking spray to prevent sticking.
Mix Wet Ingredients: In a large mixing bowl, combine the mashed bananas, Greek yogurt, olive oil, and eggs. Use a mixer on medium speed to blend until smooth.
Add Sugar: Add the brown sugar to the banana mixture and continue to mix until the sugar is fully incorporated.
Prepare Dry Ingredients: In a separate bowl, measure and mix the all-purpose flour, whole wheat flour, ground flaxseed, baking soda, salt, and cinnamon. Whisk together until evenly combined.
Combine Wet and Dry Mixtures: Gradually add the dry ingredients to the banana mixture. Gently fold in walnuts, if desired. Mix gently until just combined—avoid overmixing to keep the bread tender.
Bake: Pour the batter into the prepared loaf pan. Smooth the top with a spatula. Bake at 350°F for 55 minutes, or until a toothpick inserted into the center comes out clean. Tent with foil the last 10 minutes of baking to avoid over-browning.
Cool and Serve: Let the banana bread cool in the pan for about 10 minutes before transferring it to a wire rack to cool completely. Slice and enjoy!