Preheat your oven to 350°F (175°C) and lightly grease a small oven-safe baking dish with a touch of coconut oil or avocado oil spray.
Core and finely dice the small apple, leaving the skin on for extra fiber and nutrients.
In a medium mixing bowl, whisk together the liquid egg whites, nonfat Greek yogurt, maple syrup, and vanilla extract until the mixture is smooth and well combined.
Add the rolled oats, baking powder, ground cinnamon, ground nutmeg, and sea salt to the wet ingredients, stirring until the oats are fully submerged.
Fold in half of the diced apples into the oatmeal base.
Pour the mixture into the prepared baking dish, then top evenly with the remaining diced apples and the chopped walnuts.
Bake for 25 to 30 minutes, or until the center is set and the top is lightly golden brown.
Remove from the oven and let it cool for 5 minutes before serving warm.