Preheat the oven to 350 degrees Fahrenheit.
Combine the ground flax with 2 1/2 tbsp water in a small bowl and set aside to thicken.
Add the bananas to a blender and blend until smooth. Usually, you just mash bananas for banana bread, but with this gluten free recipe, I have the best results when blending. Once pureed, check to see that you have 1 3/4 cup worth. A little bit more is ok too, but you don't want any less.
Once pureed, add all remaining wet ingredients, including the flax egg once it has thickened, to a large mixing bowl. Whisk together.
Add in the cinnamon, salt, baking soda, baking powder and mix well.
Add in the both flours using the " pour and level" method. Pour the flour into your measuring cup and then level off the top and add what's in the cup to the bowl. Avoid measuring the flour right out of the bag since you will end up with more flour than the recipe really calls for and the bread will be a bit too dense. This is really important with gluten free baking! Stir to combine.
Once all of the flour has been added, stir in the oats and use a wooden spoon to make sure everything is well combined.
Add to a 9x5 loaf pan lined with parchment paper. An 8 1/2 x 4 1/2 works too. Bake for 30 minutes at 350, then decrease the temperature to 325 for 15-20 minutes or until a toothpick comes out clean. Careful not to over bake!
Let cool in the pan for 10 minutes, then remove from the pan and let cool for at least 2 hours before slicing to allow the bread to set. Otherwise, it will be more crumbly if you do not let it cool. I like to let it cool overnight before slicing for best results, but just a couple of hours will work too.