Preheat oven to 375 degrees. Grease or spray a 1 and 1/2-quart casserole dish with either butter or cooking spray.
In a medium bowl, toss together the oats, half of the walnuts, the baking powder, cinnamon, and salt. In a separate small bowl, whisk together the maple syrup, milk, egg, the melted butter, and the vanilla.
Place the sliced banana pieces in a single layer in the bottom of the prepared casserole dish. Sprinkle about two-thirds of the fresh blueberries over the top of the bananas, then cover the blueberries with the oat mixture.
Drizzle the milk mixture over the oats, making sure to distribute the milk as evenly as possible. Sprinkle the remaining blueberries and walnuts over the top of the oats.