Preheat your oven to 350F.
Put the diced apple in a microwave safe dish and heat on high until soft, around 2-3 minutes. You can also do this on the stovetop if you'd like!
Add the cooked apple, oats, egg & egg white, baking powder, cinnamon and salt to a blender. Blend until smooth.
Add all the ingredients to a small baking dish, I recommend either a mini dutch oven, a mini pie dish (which is what I used) or a small loaf tin.
In a small dish, whisk together all the ingredients for the optional streusel topping. Gently pour that evenly on top of the batter.
Bake for 20-25 minutes or until you can insert a toothpick and it comes out clean. Eat immediately!