Heat oven to 375 degrees F. Generously spray the inside of a 2-quart baking dish with cooking spray and place on a baking sheet. In a large bowl, mix together the oats, sugar, baking powder, orange zest, Chinese five spice, salt, half the walnuts, half the strawberries, and half the chocolate. Add to prepared baking dish. Scatter the remaining strawberries, walnuts, chocolate, and the banana on top of the oats. In another large bowl, whisk together the milk, egg, butter, and vanilla extract, then pour over the oats and fruit. Gently shimmy and shake the baking dish to help the milk mixture go throughout the oats. Bake oatmeal until the top is golden brown and the milk mixture has set, about 35 minutes. For an extra tasty top, sprinkle a tablespoon or so of brown sugar on top of the oatmeal, and then place back in the oven under the broiler and broil for 20-30 seconds.