1. Preheat oven to 375°F and generously spray the inside of a 10-1/2 by 7 inch baking dish with cooking spray and place on a baking sheet.
2. In a large bowl, mix together the oats, sugar, baking powder, cinnamon, salt, half the walnuts, half the berries and half the chocolate. (Save the other half of berries, walnuts and chocolate for the top of the oatmeal).
In another large bowl, whisk together the milk, egg, butter and vanilla extract.
3. Add the oat mixture to prepared baking dish. Arrange the remaining berries, walnuts and chocolate on top. Add the banana slices to the top then pour the milk mixture over everything. Gently shake the baking dish to help the milk mixture go throughout the oats. For an extra tasty top, sprinkle a tablespoon or so of extra brown sugar. Bake 35 to 40 minutes or until the top is nicely golden brown and the milk mixture has set.
I prefer dark chocolate chips and black cherries!