1. In a small bowl combine the yeast, one tablespoon of sugar, and warm water. Allow the yeast to proof for five minutes.
2. In a large mixing bowl whisk together four cups of the oat flour with the tapioca flour, potato starch, xanthan gum, sugar, and salt. Make a small well in the center of the dry ingredients. Place the eggs, canola oil, seltzer, and yeast into the well. Mix until the ingredients are just smooth and combined.
3. Allow the dough to rest for two minutes. If the dough is particularly sticky or loose, add the remaining half cup of oat flour and mix until the dough is smooth.
4. Spoon the dough into a braided loaf pan, or form small dough balls and place into a standard loaf pan to form braids. (You can also drop the balls into a muffin pan to make rolls.) Cover the loaf and let rise for one hour.
5. Combine the large egg and warm water and brush over the risen loaf. Top with sesame or poppy seeds or dehydrated onion if desired.
Meanwhile, preheat the oven to 350 degrees Fahrenheit. When the challah has risen, bake for 30 minutes, or 20–25 minutes for rolls, until the top is golden brown.