Preheat oven to 350 F. Make your flax-eggs by combining the ground flaxseed and water in a small bowl. Mix together and set aside. In a larger bowl mash your bananas with a fork or potato masher.
Add the melted butter, milk, vanilla, apple cider vinegar and sugar. Mix until everything is combined. Now, add the baking soda, baking powder and salt. Mix that together and then add the oat flour.
In a 9x5 pan, either line it with parchment paper or lightly oil. Pour the batter in and spread evenly. Top with nuts or more banana. Gently press them in to the dough. Bake for 45-50 minutes. Time varies depending on your oven. Make sure to check it with a tooth pick.
Allow to complety cool before serving. Store in the fridge in an air-tight container. It will be good for 3-4 days in the refrigerator. Freeze it for longer.
If the bananas are small use 4 instead of 3.
You can use coconut sugar in place of date sugar.
If you don't have oat flour and still want to keep it gluten-free, use 2 cups GF All Purpose Flour and add 1 1/2 tsp of xanthan gum.