Preheat your oven to 375°F (190°C) and lightly grease a small oven-safe baking dish or 16-ounce ramekin with a touch of coconut oil.
In a medium mixing bowl, mash half of the banana with a fork until it reaches a mostly smooth consistency.
Add the rolled oats, liquid egg whites, non-fat Greek yogurt, ground cinnamon, vanilla extract, sea salt, and baking powder to the bowl with the mashed banana.
Whisk the ingredients together until thoroughly combined and the oats are well-submerged in the liquid mixture.
Pour the mixture into your prepared baking dish, smoothing the top with a spoon to ensure even baking.
Slice the remaining half of the banana into thin rounds and arrange them on top of the oat mixture.
Sprinkle the chopped walnuts over the top and drizzle evenly with the maple syrup to encourage a deep caramelization.
Bake for 25 to 30 minutes, or until the center is set and the banana slices have turned a beautiful golden brown.
Let the bake cool for 5 minutes before serving to allow the texture to firm up and the flavors to meld.