Prepare oat crumble: In a medium bowl, stir together oats, flour, 3 tbsp maple sugar and salt until combined. Add butter and stir until moistened. Set aside.
Mist a 9 x 13-inch baking dish with cooking spray. Arrange 7 slices bread in a single layer (2 rows of 3 1/2 slices each). Top with banana slices in a single layer, then top with remaining 7 slices bread in a single layer. In a large bowl, whisk eggs. As you whisk, slowly pour in milk until combined. Whisk in vanilla, cinnamon and remaining 2 tbsp maple sugar. Pour mixture over bread, then sprinkle evenly with oat crumble. Cover with foil and refrigerate overnight.
Preheat oven to 350°F. Bring casserole to room temperature, up to 2 hours. Remove foil and bake until bread is puffed up and egg mixture is set (a bit of liquid in center is fine as it will firm up as it cools), about 40 minutes. (NOTE: If you skipped bringing casserole to room temperature, bake another 10 to 15 minutes.) Cool on a rack for 10 minutes before serving.
If you think your oatmeal has no place in the cold breakfasts of warm summer mornings, pull it back out! Cool and creamy overnight oats layered with frosty fruit purée is kind of like eating ice cream for breakfast, but with a lot less guilt.