Preheat oven to 350 F degrees. Line a pie plate or 8 x 8 baking dish with parchment paper.
In a large bowl, mash two of the bananas (you should have 1 ½ cups). Slice the third banana lengthwise and set aside.
To the banana bowl, add the oat or almond milk, maple syrup, eggs, melted coconut oil, and vanilla. Add the oats, baking powder, cinnamon, and sea salt; Mix well.
Fold in the walnuts. Transfer to the prepared baking dish and spread evenly.
Bake for 50 minutes or until the oat mixture is set and the top is golden. Remove from the oven, and allow to cool for a few minutes before serving.