Place all the starter ingredients in a medium-size bowl, and whisk until well-combined. The mixture will be thick and shapeless.
Cover the bowl with plastic wrap, and set it aside in a warm, draft-free location to rise until at least doubled in size.
This will take about an hour in a relatively warm environment, but could take much longer in a cold, dry kitchen.
Make the dough
Once the starter has finished rising, in the bowl of a stand mixer fitted with the paddle attachment or a large bowl with a heavy-duty handheld mixer, place the flour, xanthan gum, tapioca starch/flour, salt, and sugar, and whisk to combine well.
Add the risen starter and the 3 tablespoons olive oil and mix on low speed until combined.
Raise the mixer speed to medium-high and mix until the dough begins to take on a whipped appearance (about 3 minutes).
For the cold rise
Transfer the dough to a lightly oiled proofing bucket or bowl with a lid that’s large enough for the dough to double (although it won’t likely double).
Cover the bowl or bucket tightly with the lid (if you don’t have a lid make sure you cover it very well with plastic wrap) and place it in the refrigerator for at least 12 hours and up to 3 days.
Shape and top the dough
On baking day, begin with a rimmed baking sheet that’s about jelly roll pan size (10-inches x 15-inches). A half baking sheet (13-inches x 18-inches) is fine, too.
Drizzle about 2 tablespoons of the olive oil from the topping on the bottom of the baking sheet, and spread it evenly around the surface of the pan.
Remove the prepared dough from the refrigerator and turn it out onto the oiled baking sheet.
Drizzle the top and your clean hands lightly with a bit more oil, and, with the tips of your fingers, press and push the dough into a rectangle about 9-inches x 13-inches.
The dough will be very sticky and difficult to handle. Just keep your hands oiled and use a light touch.
Using an oiled rolling pin, an oiled bench scraper, or an oiled cake turner, smooth the top of the rectangle as much as you can.
Sprinkle the top of the dough evenly with the (optional) dried herbs. Drizzle evenly with the remaining olive oil, brushing it very gently with a pastry brush to distribute the oil evenly.
For the second rise
Cover the top of the dough with oiled plastic wrap and place it in a warm, draft-free location to rise until nearly doubled in size (at least 2 hours). The dough will rise both up and out.
Bake the bread.
When the dough is nearly at the end of its rise, preheat your oven to 400°F.
Uncover the risen dough, drizzle lightly with more olive oil if you like, and place in the center of the preheated oven. Bake for 15 minutes.
Reduce the oven temp to 375°F and bake until most of the oil has been absorbed by the bread, the top is lightly golden brown, the bread is firm to the touch, and the internal temperature with an instant-read thermometer in the center reads at least 200°F (another 10 to 15 minutes).
Remove the pan from the oven, and immediately transfer the bread to a wire rack to cool for about 20 minutes. Slice and serve warm.
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Notes
For the flours.My favorite gluten free flour blends are Better Batter's original blend gluten free flour and Nicole's Best multipurpose blend. Be sure to add 1 teaspoon xanthan gum per cup of flour if you're using Nicole's Best. Bob's Red Mill 1-to-1 Gluten Free Baking Flour should also work, but you'll need to add an additional 1/4 teaspoon xanthan gum to the starter and an additional 1 teaspoon to the dough dry ingredients.Cup4Cup changed its formula and doesn't seem to work as well as it has in the past, so I don't recommend it. To make your own blend using one of my “mock” recipes, please see the all purpose gluten free flour blends page.Caputo Fioreglut and King Arthur Flour Gluten Free Bread Flour may also work in this recipe. They will absorb more moisture, so the bread will have smaller holes, but I think the recipe will probably still work.For the oil.Olive oil adds fat, helps the dough crisp in the oven, and adds a lot of flavor. If you'd like to use another oil, just make sure it's something that tastes and smells good to you.