Preheat the oven to 350F, and prepare an 8x4-inch loaf pan with cooking spray or parchment paper. Set aside.

Optionally, blend the psyllium husk and flaxseed meal in a blender for about 30 seconds, until they're extra finely ground.

Transfer the psyllium husk and flaxseed meal to a large mixing bowl, along with the almond flour, salt, baking powder, and xanthan gum. Whisk to combine.

Whip the egg whites in an electric stand mixer fitted with a whisk attachment on high speed, until stiff peaks form, about 5-8 minutes.

Mix the apple cider vinegar into the boiling water, and pour it over the ingredients. Mix quickly until a thick dough forms, about 1 minute.

Working quickly, add about 1 cup of the whipped egg whites to the bowl of dough, and mix vigorously until the dough becomes looser.

Fold the remaining whipped egg whites in 2 more additions, being careful not to mix. Otherwise the egg whites will deflate. Fold the batter enough that it is a uniform color and consistency. This may take 2-4 minutes.

Transfer the batter to your prepared loaf pan and spread into an even layer.

Bake in the center rack of the oven for 70-80 minutes. The outside of the bread should form a nice browned crust.

Allow the bread to cool for about 1 hour before slicing.