Preheat oven to 375°F and spray olive oil in your baking pan to prevent sticking. Mix together oats, quinoa, half of walnuts, sugar (if using otherwise add maple syrup with wet ingredients), baking powder, cinnamon, and salt.
In a separate bowl, lightly beat egg and add in milk, vanilla, maple syrup (if using) and half of the slightly cooled butter. Place banana slices on the bottom of the prepared pan. Top with 2/3 of the berries and cover with oat mixture. Pour wet ingredients over and make sure the dry ingredients are covered. Top with remaining berries and walnuts.
Bake for 40 minutes. Drizzle with melted butter and additional maple syrup or sugar if desired.