In a large mixing bowl whisk together the milk, eggs, brown sugar and cinnamon until fully combined.
Stir in the oats, baking powder and salt ensuring the oats are incorporated into the milk mixture. Stir in the diced apple.
Pour the oatmeal into an 8x8 inch baking dish prepared with cooking spray.
Transfer to the oven and bake for 40 minutes or until the center is set and the oatmeal is golden on top. Allow to cool for at least 10 minutes before slicing and serving.