Preheat the oven to 375°F. Spray the inside of an 8-inch square baking dish with cooking spray.
In a bowl, mix together the oats, baking powder, cinnamon, salt, and half the walnuts.
In another bowl, whisk together the maple syrup, milk, egg, butter (slightly melted), and vanilla.
Slice bananas in 1/2-inch pieces and arrange in a single layer in the bottom of the prepared baking dish.
Sprinkle two-thirds of the berries over top of the bananas, then cover with the oat mixture.
Slowly drizzle the milk mixture over the oats, then shake the dish gently to make sure the milk moves all the way through the dish.
Top with remaining berries and walnuts.
Bake for 35 minutes until the top is nicely golden and the oat mixture has set.
Remove from the oven and let cool for a few minutes before slicing into 6 pieces.