Preheat oven to 375 degrees. Spray inside of 10.5x7 baking dish with cooking spray. Mix together oats, sugar, baking powder, orange zest, cinnamon, salt, half the walnuts, half the strawberries, and half the chocolate. Add the oat mixture to the prepared dish. Arrange the remaining strawberries, walnuts, and chocolate on top. In another large bowl, whisk together the milk, egg, butter and vanilla extract. Pour the milk mixture over everything in dish. Gently shimmy the baking dish to help the milk mixture soak throughout the oats. Bake 35-40 minutes or until the top is golden brown. Serve warm with an extra sprinkle of sugar or drizzle of maple syrup.