Preheat oven to 350 degrees. Grease a 9x5-inch loaf pan; set aside.
In a small bowl, stir together flax seed and water. Set aside for 10-15 minutes or until thickened and the consistency of an egg.
In a separate bowl or measuring cup, add the lemon juice or vinegar to the milk. Allow to sit for 5-10 minutes or until "soured".
In a large bowl, whisk together the flours, baking soda, and salt.
Whisk together the "flax egg", soured milk, mashed banana, brown sugar, coconut oil, and vanilla. Add to the dry ingredients and mix just until combined. Fold in the millet, then pour the batter into the prepared loaf pan. Bake for 35-45 minutes or until a toothpick inserted into the center comes out clean. Allow to cool in the pan before removing to a baking rack.