Preheat oven at 350ºF.
In a large bowl, add flour, oats, flax, chia, baking powder, baking soda, and salt. Mix well with a spatula.
In another bowl, add brown sugar with 1/2 cup water. Mix so the sugar starts to dissolve. Add the bananas. Mash them until they are lumpy, but not pureed. Add the melted butter, egg, and vanilla. Mix well.
Make a well with the dry mix. Pour the wet ingredients into the well. With the spatula mix the ingredients together until they are incorporated. Do not over mix.
Fold in the walnuts.
Spoon batter into a lined muffin tray. Bake for 30 minutes.