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.

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).

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.

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.

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.

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.

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).