Make the psyllium gel: In a bowl, mix together the psyllium husk and lukewarm water. After about 30-45 seconds, a gel will form.
In a large bowl or the bowl of a stand mixer, whisk together the tapioca starch, millet flour, oat flour, sugar, instant yeast, baking powder, xanthan gum and salt.
Add the egg, two egg yolks and oil to the psyllium gel and mix well to combine.
Make a well in the middle of the dry ingredients and add the psyllium gel-egg mixture.
Knead the dough until smooth and all the ingredients are evenly incorporated.
Turn out the dough onto a lightly floured surface and shape it into a ball, then divide it into four equal portions.
Shape each portion into a ball, roll it out into a rectangle, and then roll it up into a log.
Roll the dough into 15-16 inch ropes and braid them together.
Transfer the braided challah onto a baking sheet and proof in a warm spot until doubled in size, about 1 hour to 1 hour 15 minutes.
Preheat the oven to 375ºF (190ºC).
Brush the challah with egg wash and sprinkle with sesame seeds.
Bake for 28-30 minutes until the crust is deep golden brown.
Transfer the baked challah to a wire rack to cool.
