Put the coconut oil in the mug and microwave for 15-20 seconds until melted.
Cut the peeled banana into chunks and place in the mug. Using a fork, mash it against the side of the mug until its blended into a puree.
Add to the mug the coconut flour, almond flour and baking soda. When adding the apple cider vinegar try to ensure that it makes contact with the baking soda (the baking soda will fizz when it touches the apple cider)
Add in the vanilla extract and egg and walnuts. Stir until well combined.
Microwave the mug for 2 1/2 to 3 minutes or until the banana bread has risen and the top is fully cooked and no longer wet to the touch. Serve topped with chopped walnuts or banana chips.