Preheat oven to 375F. Generously butter the inside of an 8-inch baking dish. In a bowl, mix together the oats, half the walnuts, baking powder, cinnamon and salt. In another bowl, whisk together the maple syrup, milk, egg, the butter, and the vanilla. Arrange the bananas in a single layer in the prepared pan. Sprinkle two-thirds of the berries over the top, cover the fruits with the oat mixture. Slowly drizzle the milk mixture over the oats. Make sure the milk covers the oats. Scatter the remaining berries, remaining walnuts and cranberries across the top. Bake for 35-40 minutes or until the top is nicely golden and the oat mixture has set. Drizzle with maple syrup if desired.