Preheat oven to 375 °F. Spray an 8x8-inch casserole dish with nonstick spray.
In a large mixing bowl, whisk together the applesauce, egg and egg whites until eggs are fully beaten.
Whisk in the sugar, vanilla extract, baking powder, salt and cinnamon
Stir in the milk
Stir in the oats
Stir in the sliced nectarines and blackberries
Pop the whole thing in the oven and bake for 45 minutes. If you're using frozen fruit, you might need to bake it a little longer (about an hour), so keep an eye on it! You'll know it's done when a skewer or knife inserted into the center of the oatmeal comes out clean.