Whisk tapioca starch, sorghum flour, xanthan gum, and salt together in a large mixing bowl.
Mix in sourdough starter, milk, honey, oil, and beaten egg. Beat the batter together for 7 minutes using a large wooden spoon. Alternatively, you can blend all ingredients in a stand mixer and beat with the paddle attachment for five minutes.
Generously butter a baking sheet. Divide dough into two parts. Prepare a small bowl of water for moistening your hands as needed in working with the sticky dough. Use your hands to scoop half of dough and form it into a 14-inch log. It should end up 2 to 3 inches wide and a little over 1 inch high. Repeat with the second half of the dough.
Use a sharp knife to cut diagonal slashes on the log every 1-1/2 inches. Place a damp towel over dough very gently and let rise in a warm place for 6 to 8 hours, or overnight. Dough may or may not rise significantly. There will be a significant rise in the oven, so do not worry.
Preheat oven to 425°F. When hot, place bread in oven and bake for 30 to 35 minutes or until internal temperature reads at least 190°F.
Remove from oven and transfer to cooling rack after a few minutes. Allow to cool at least 10 minutes before serving.