Baked Blueberry & Banana Oatmeal

oats 1 teaspoon baking powder 1 1/2 teaspoons cinnamon 1/2 teaspoon salt 1/3 cup sugar or maple syrup 1/2 cup walnuts; chopped 2 cups milk 1 tsp vanilla extract 1 egg 2 Tbsp flax seeds (ground or flax seed meal) 2 cups fresh blueberries, divided 2 banana