Preheat your oven to 350°F (175°C) and lightly grease a small oven-safe dish or mini loaf pan with a touch of oil.
In a medium mixing bowl, mash the banana with a fork until it reaches a smooth consistency.
Add the egg, egg whites, vanilla extract, and 0.5 cup of the Greek yogurt to the banana, whisking until well combined.
Fold in the oat flour, ground cinnamon, baking powder, and sea salt until a thick batter forms.
Stir in the maple syrup and half of the chopped walnuts into the mixture.
Transfer the batter into the prepared dish and sprinkle the remaining walnuts over the top.
Bake for 25 to 30 minutes, or until the center is firm and the edges are golden brown.
Allow the bread to cool slightly before serving it warm with the remaining 0.5 cup of Greek yogurt on the side.