Preheat oven to 375F with rack in top third of oven. Butter an 8x8 inch baking pan.
In a bowl mix together oats, half the walnuts, sugar, if using, baking powder, cinnamon. In another bowl, whick together maple syrup, if using, milk, egg, half the butter and vanilla.
Arrange bananas in a single layer in bottom of prepared dish. Sprinkle two thirds of berries over top. Cover fruit with oat mixture. Slowly drizzle milk mixture over oats. Gently give baking dish a few thumps on the counter to make sure milk moves through oats. Scatter remaining berries and walnuts on top.
Bake 30-45 minutes until top os golden and oats have set. Remove from oven and drizzle remaining butter on top. Serve with extra sugar or syrup!