In the bowl of a stand mixer or large mixing bowl, add the flours, salt, and xanthan gum. Whisk to combine.
In a 4 cup measuring cup or medium bowl, mix the water with the sugar. Stir until almost dissolved.
Add the yeast and set aside to proof for 5 minutes.
In the mixing bowl, with the flours, add in the proofed water-yeast mixture and the olive oil. Mix on low speed for 30 seconds. Stop the machine and scrape down the sides of the bowl.
Turn the mixer to medium speed and beat for 9 minutes.
Scrape batter into a bowl and cover tightly with a lid. Refrigerate the batter for at least 6 hours.
When ready to bake, place a large sheet of parchment paper on the counter. Slice off ⅓ of the bread batter (it will be slightly sticky).
Place the bread batter in the middle of the parchment paper. Place the parchment paper in a large pot and cover with the lid to let rise until 50% larger.
Preheat oven to 450F. While the oven is preheating, place a Dutch oven with the lid on, into the oven to heat. See the instructions below for preparation without a Dutch oven.
Once the bread is ready to bake, use potholders to very carefully remove the pot from the oven. Place the bread, parchment paper and all, from the large pot into the hot Dutch oven. Put the lid on the Dutch oven.
Bake at 450F for 30 minutes. After 30 minutes, remove the lid and continue baking for about 10 minutes.
Test the bread for doneness with an instant-read thermometer (it should register 205F).
Remove the baked bread from the oven and very carefully lift the parchment paper from the Dutch oven to a wire rack to cool.
Let the bread cool for at least 4 hours before slicing.
Notes
Make sure all ingredients are at room temperature - water slightly warmer.
To easily portion out 3 loaves, alternate dropping spoonfuls of batter on the pan.
Bake these loaves using a water bath for a wonderful crusty exterior.
Before the oven gets too hot, move the racks so one is about 3 inches from the bottom one. Place a baking sheet on the bottom one and fill the baking sheet with water. (It should reach about halfway up the sides of the sheet).