Preheat oven to 375 degrees F. Spray cooking oil in 8×8” baking dish; set aside.
Cut bananas into ½” slices and arrange in a single layer on the bottom of the baking dish. Sprinkle half of the blueberries, ¼ tsp of the cinnamon, 1 tbsp of the honey over the bananas and cover with foil. Bake for 15 minutes, until the bananas get soft.
Meanwhile, in a medium bowl, combine oats, half of the nuts, baking powder, remaining cinnamon, and salt. Stir together.
In a separate bowl, mix together the remaining honey, milk, egg, and vanilla extract.
Remove the bananas from the oven and pour the oat mixture evenly over the bananas and blueberries.
Pour the milk mixture over the oats, making sure it is distributed as even as possible over the oats. Sprinkle the remaining blueberries and walnuts over the top.
Bake the oatmeal for about another 30 minutes, or until the top is golden brown and the oatmeal has set. Serve warm from the oven.