Preheat your oven to 350°F and lightly grease a small loaf pan with a small amount of coconut oil or parchment paper.
In a large mixing bowl, mash the ripe banana with a fork until it reaches a smooth, liquid-like consistency.
Add the egg whites, non-fat Greek yogurt, and vanilla extract to the banana, whisking vigorously until the mixture is fully combined and aerated.
In a separate bowl, whisk together the oat flour, collagen peptides, ground flaxseed, Ceylon cinnamon, baking powder, and sea salt to ensure no clumps remain.
Gradually fold the dry ingredients into the wet mixture using a spatula, stirring gently until just incorporated to keep the loaf light and airy.
Pour the batter into the prepared loaf pan and smooth the top with your spatula.
Roughly chop the walnuts and sprinkle them evenly over the surface of the batter, pressing them in slightly so they adhere.
Bake for 35 to 40 minutes, or until the top is golden brown and a toothpick inserted into the center comes out clean.
Remove from the oven and let the loaf cool in the pan for at least 10 minutes before transferring it to a wire rack to cool completely.