First, combine the almond flour, chia seeds, psyllium husk powder, sea salt, and baking powder. You can combine the ingredients in a small bowl and then transfer to a coffee mug. Alternatively, you can combine all the ingredients directly into a tall coffee cup (about 12 oz).
Combine boiling water and apple cider vinegar. Add this to the mixture as well. Mix extremely well to incorporate any dry pieces.
Add in the egg. Mix very well. The mixture will be very thick at this point. If using a bowl to mix, transfer the ingredients to a tall coffee mug (about 12 oz).
Cook for 90 seconds in the microwave. Add an extra 30 seconds if necessary. Carefully remove the mug bread from the coffee cup onto a cutting board. Allow to cool thoroughly. Then cut the mug bread into 3-4 slices and enjoy immediately.