Preheat your oven to 350°F and lightly grease a standard small loaf pan with avocado oil.
In a large mixing bowl, mash the overripe bananas with a fork until they reach a mostly smooth consistency.
Stir in the liquid egg whites, non-fat Greek yogurt, and vanilla extract until the wet ingredients are well combined.
Add the vanilla whey protein powder, oat flour, baking powder, ground cinnamon, and sea salt to the bowl.
Mix gently until the dry ingredients are just incorporated, being careful not to overwork the batter.
Pour the batter into the prepared loaf pan and sprinkle the crushed walnuts evenly across the top.
Bake for 35 to 40 minutes, or until the center is set and a toothpick comes out clean.
Allow the bread to cool in the pan for at least 10 minutes before slicing into thick, warm pieces.