Preheat oven to 375°F. Lightly spray a 9 x 9" ceramic dish with non-stick cooking spray.
Arrange the banana slices in a single layer on the bottom of the ceramic dish. Sprinkle half of the blueberries (1 cup) and 1/4 tsp of the cinnamon over the bananas and cover with foil. Bake 15 minutes at 375°F, until the bananas get soft.
Meanwhile, in a medium bowl, combine the oats, half of the nuts (1/4 cup), baking powder, remaining cinnamon (1/2 tsp), chia seeds and salt; stir together.
In a separate bowl, whisk together the agave, milk and vanilla extract.
Remove the bananas from the oven. Then pour the oat mixture over the baked bananas and blueberries.
Pour the milk mixture over the oats, making sure to distribute the mixture as evenly as possibly over the oats. Sprinkle the remaining blueberries (1 cup) and walnuts (1/4 cup) over the top.
Bake the oatmeal for about 30 minutes, or until the top is golden brown and the oatmeal has set. Serve warm from the oven.