Preheat the oven to 375 degrees F. Spray an 8-inch square glass or ceramic baking dish with nonstick cooking spray. You want the mixture to be 3 to 4 inches high in the dish so it doesn't dry out.
Combine the oats, 1 tablespoon of the dark brown sugar, baking powder, cinnamon, nuts, if using, and salt in a large bowl.
Mash 1/2 of the banana in a medium bowl. Whisk in the milk, melted butter, vanilla and egg until combined.
Stir the wet ingredients into the dry ingredients until fully incorporated. Very gently fold in the blueberries with a rubber spatula.
Pour the oatmeal into the prepared baking dish and smooth the top. Thinly slice the remaining 1/2 banana crosswise and arrange the slices in an even layer over the top. Sprinkle with the remaining 1 tablespoon dark brown sugar (see Cook's Note). Bake until the top is slightly browned and the oats are tender, 35 to 45 minutes. Enjoy!
Adding the brown sugar on top before baking is optional.