Preheat oven to 325°F.Spray an 8 x 8-inch baking dish with cooking spray. Finely chop ½ cup walnuts and place them in a bowl along with the oats, baking powder, cinnamon and salt.Whisk the milk, yogurt, egg, maple syrup, coconut oil and vanilla together in another bowl.Arrange 1 cup blueberries and 1 cup chopped apple in the bottom of the prepared baking dish. Scatter the oat mixture evenly on top. Pour in the milk mixture and press to submerge all of the dry ingredients in the liquid. Roughly chop the remaining ½ cup walnuts and scatter them on top along with the remaining ½ cup blueberries and ½ cup apple. Sprinkle with cinnamon.Bake 40-45 minutes, until the top is golden and the oats are set. Serve warm or at room temperature. Serve as is or topped with milk, Greek yogurt or maple syrup.