Preheat the oven to 350 degrees.
Spray your 8x8 baking dish with nonstick spray.
In a large bowl, thoroughly whisk together the oats, baking powder, cinnamon, salt, and sugar.
In a smaller bowl, whisk together the egg, milk, and applesauce.
Add your wet ingredients to the large bowl of dry ingredients and stir to combine.
Fold in the diced apples, reserving 1/4 cup for topping your oats.
Transfer the mixture to the prepared baking dish. Sprinkle the remaining diced apples over the top. Alternately, you could top your oats with thinly sliced apples.
Bake at 350 degrees for about 45 minutes or until a toothpick inserted into the center comes out clean.
Allow to cool a bit before serving, but serve warm. Top with any additional toppings you prefer.