Preheat the oven to 350 degrees Fahrenheit.
In a large mixing bowl, mash the banana with a fork.
Add in the egg, oil, nut butter, maple syrup, vanilla, milk and apple sauce.
Whisk to combine.
Add in the rolled oats, salt, cinnamon, nuts and baking powder. Mix well.
Add 1/2 cup of blueberries to the bottom of a 9x9 baking dish. Add the remaining 1 cup to the bowl with the oat mixture and stir to combine.
Transfer the mixture to the baking dish and carefully stir to ensure the blueberries are mixed evenly throughout. Top with a few additional blueberries, chopped nuts or sliced bananas if desired.
Add to the oven, uncovered, for 35-45 minutes or until golden brown around the edges and the center doesn't look too wet. If you want the center to be more cakey, leave it in the oven for longer, but if you like a more creamy consistency, take it out around 35-40 minutes.
Remove from the oven and serve warm with additional maple syrup, yogurt or just as is!