Preheat oven to 180°C.
Spray a baking dish with non-stick spray.
Place walnuts in a bowl with whole rolled oats, baking powder, cinnamon and salt.
Whisk the milk, yoghurt, maple syrup, butter and vanilla 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.
Scatter the remaining ½ cup chopped walnuts on top with the remaining ½ cup blueberries and ½ cup apple. Sprinkle with cinnamon.
Bake 40-45 mins until the top is golden and oats are set.
Serve warm or at room temperature.
Serve as is or topped with milk, yoghurt or maple syrup.