Preheat the oven to 350°F and 8×8-inch or similar baking dish.
In a large bowl, whisk together the mashed banana, almond milk, almond butter, and apple sauce. Add the baking powder, cinnamon, and salt and whisk again until combined.
Fold in the oats, ¾ cup of the walnuts, and ½ cup of the blueberries. Pour the mixture into the 8×8 baking pan and smooth into an even layer.
Top with the remaining ½ cup blueberries, the remaining ¼ cup walnuts, and brown sugar.
Bake for 40 to 50 minutes, or until the topping is crisp and the middle is set. Remove from the oven and let cool for 10 minutes before serving.