Preheat oven to 350 degrees F (176 C). Line an 8.5 x 4.5 inch loaf pan with parchment paper. The paper doesn't have to cover the ends; just make sure there's enough overhang to easily lift the loaf from the pan after baking.
In a large mixing bowl whisk together the oat flour, baking powder, baking soda, cinnamon, nutmeg, and salt.
In a medium-size bowl, mash the bananas until mostly pureed (a few larger bits are okay). Measure to make sure you have 1 ⅓ cups. If you have extra banana, either eat it, discard it, or set it aside to drizzle on top of the loaf just before baking.
To the 1 ⅓ cups mashed banana add maple syrup, coconut sugar, nut butter, milk, vinegar, and vanilla. Stir very well until nut butter is completely incorporated. Pour wet ingredients into dry, and stir to combine. Pour batter into pan. If you had extra banana puree, place dollops of it on top of the batter, if desired. Arrange nuts on top - feel free to chop the nuts or break them up with your hands. I like to leave a few whole for appearance.
Bake for 55 to 65 minutes or until a tooth pick inserted in the center of the loaf comes out clean. Allow to cool in the pan for 15 to 20 minutes, then use the parchment to lift the loaf out onto a cooling rack. Allow to cool completely before slicing (about 1 hour).
Measuring Oat FlourThere are contradictory weights for oat flour depending on the package or online source. I have measured and weighed Arrowhead Mills organic oat flour (90 g per cup) and my own oat flour I blended in the Vitamix with similar results. Some sources indicate that a cup of oat flour weighs up to 130 grams - ignore that!For the best results use a kitchen scale and weigh exactly 180 grams oat flour for this recipe. If you don't have a scale, whisk the oat flour first, then spoon it into a measuring cup and level off the top. If you use a measuring cup to scoop the oat flour straight out of a bag, you'll likely scoop up too much which will result in a dry bread.StorageStore banana bread in an airtight container at room temperature for up to 2 days or refrigerated for up to 5 days. Can also be frozen.