Preheat your oven to 350°F (175°C) and lightly grease a 6x3-inch mini loaf pan with a small amount of avocado oil.
In a medium-sized mixing bowl, mash the half banana with a fork until it reaches a smooth consistency with very few lumps.
Whisk the non-fat Greek yogurt, egg whites, and vanilla extract into the mashed banana until the wet ingredients are fully combined.
Sift the oat flour, vanilla protein powder, baking powder, ground cinnamon, and sea salt into the bowl with the wet ingredients.
Gently fold the dry ingredients into the wet mixture using a spatula until just combined, ensuring you do not overmix the batter.
Roughly chop the walnuts and stir half of them into the batter, saving the rest for the topping.
Pour the batter into the prepared mini loaf pan and smooth the top with your spatula.
Sprinkle the remaining chopped walnuts evenly over the top of the batter for a toasted finish.
Bake for 30 to 35 minutes, or until the top is golden brown and a toothpick inserted into the center comes out clean.
Allow the loaf to cool in the pan for at least 10 minutes before transferring it to a wire rack to cool completely.