Preheat oven to 375 with rack in top third of oven. Butter 8x8 baking pan. In a bowl, mix together oats, half of nuts, sugar, baking powder, cinnamon, and salt. In another bowl whisk together maple syrup, if using, milk, eggs, half the butter and vanilla. Arrange bananas in single layer in bottom of baking dish. Sprinkle 2/3 of berries over top. Cover fruit with oat mixture. Slowly drizzle milk mixture over oats. Gently give baking dish a few thwacks on countertop to make sure milk moves through oats. Scatter remaining berries and nuts on top. Bake 35-45 min., until top is golden and oats have set. Remove from oven; drizzle remaining melted butter on top. Serve with extra sugar or syrup.
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