Baked Oatmeal with Fruit

old fashioned rolled oats

chopped walnuts or pecans, lightly toasted, divided

baking powder

ground cinnamon

salt

maple syrup

milk

large egg, lightly beaten

unsalted butter, melted and cooled slightly

vanilla extract

ripe bananas, peeled and sliced ½-inch thick

blueberries (fresh or frozen), divided