Add the lukewarm water to a large bowl. Whisk in the ground psyllium husk and let it sit for 1 to 2 minutes until a thick gel forms.
Add the sourdough starter, egg whites, vinegar, olive oil, and honey or maple syrup. Whisk until fully combined.
In a separate bowl, whisk together the millet flour, tapioca starch, sorghum flour, potato starch, and salt.
Add the dry ingredients to the wet mixture. Beat until fully combined and no dry patches or lumps remain.
Scrape down the bowl and form the dough into a rough ball. Cover with a damp kitchen towel and rest at room temperature for 1 hour.
Transfer the dough to a work surface lightly dusted with white rice flour. Gently shape it into a ball without overworking it.
Dust a 7-inch banneton with white rice flour. Place the dough inside with the smooth side facing down.
Cover and rest at room temperature for 1 hour. Refrigerate for 12 to 16 hours.
Place a lidded cast-iron Dutch oven inside the oven. Preheat to 500°F for 30 minutes.
Turn the chilled dough onto parchment paper and score the top.
Lower the dough into the hot Dutch oven using the parchment paper. Cover and bake at 500°F for 40 minutes.
Remove the lid and reduce the temperature to 450°F. Bake uncovered for 30 to 40 minutes, until the center reaches 207°F.
Transfer the loaf to a wire rack and cool completely before slicing.