Preheat your oven to 350°F (175°C) and lightly grease a small oven-safe ramekin or baking dish with a touch of ghee.
Core and dice the apple into small bite-sized pieces.
Heat a small skillet over medium heat, add the ghee, diced apples, and half of the cinnamon, sautéing until the apples are softened and golden.
In a medium mixing bowl, whisk together the egg whites, Greek yogurt, and maple syrup until smooth and frothy.
Fold in the oat flour, vanilla protein powder, baking powder, sea salt, and the remaining cinnamon until a thick batter forms.
Stir half of the caramelized apples into the batter, then pour the mixture into the prepared baking dish.
Top the batter with the remaining caramelized apples and the chopped walnuts to create the crumble layer.
Bake for 20-25 minutes, or until the center is set and the top is slightly golden brown.
Allow the cake to cool for 5 minutes before serving directly from the dish.