Peel & dice the Japanese or white sweet potato and put in a small saucepan. Cover with water and simmer for 15 minutes until soft (if you can easily press a fork into it and break it it is done)
Drain the sweet potato through a sieve, reserving the water for later. Mash the sweet potato in the saucepan and set both aside to cool.
In a small bowl, add 1/4 cup of the water, sprinkle with yeast and add maple syrup or honey. Set aside for 10 minutes for the yeast to activate and consume all the sugars.
In a large bowl, add the cassava flour, arrowroot, tigernut flour, salt and baking soda and mix together.
Add the olive oil, work it into the flour and then make a well in the center to start adding the wet ingredients.
Add the mashed Japanese sweet potato and the cassava sourdough starter to the center and gently fold together without mixing in too much of the flour.
In the small bowl, add the rest of the water, the apple cider vinegar and the psyllium husk and whisk. Let set for 2 minutes.
Add apple cider vinegar/yeast/psyllium husk mixture into the well and begin to stir into the sweet potato & cassava sourdough starter until it looks like a foamy sponge.
Begin folding the flour into the sponge until it is biscuit consistency. Then press together, knead it gently and form it into a boulé or baguette shape to rise.
Oil a bowl and set in the boulé to sourdough for 12-24 hours.
Alternatively, I like to roll a couple tea towels into a 6x9 pan and lay the third towel inset into it, dust it with arrowroot flour and lay the dough in the middle to rise in a sort of baguette shape. Then I dust the top with arrowroot again and fold the tea towel over the top to rise/ferment for 12-24 hours.
The next day, preheat a cast iron pan or pizza stone in the oven to 450°F
Transfer the boulé or baguette to the oven, score the top with a knife and close the door.
Turn the oven down to 375°F and bake for 45 minutes to an hour (depending on your oven)
At 45 minutes, if the loaf has browned, check the bottom by tapping. If it sounds hollow then it is done, if not, put it in for another 10-15 minutes.
Let cool at least 15 minutes before slicing.