Gather the ingredients.
Weigh the ingredients for the poolish into a Tupperware type container with a lid. Mix to combine, then cover and leave to ferment at an ambient temperature of 65 to 70 F for 12 to 16 hours.
After at least twelve hours, weigh the ingredients for the dough into a roughly 2 1/2 quart mixing bowl. (You want to end up with a dough temperature of about 75 F, so if it’s very cold or very warm in your environment, you may want to compensate by using slightly warmer or cooler water.) Add the poolish and mix with a spoon until well-combined. Cover the bowl. One efficient and effective method is to place the entire bowl into a plastic produce bag from the grocery store.
After about 45 minutes, fold the dough. Repeat two more times at 45-minute intervals for a total of three folds.
About 45 minutes after the final fold—roughly 3 hours after mixing—prepare a cutting board with a good dusting of both spelt flour and brown rice flour. Dust your countertop with spelt flour and invert the bowl so the dough falls gently onto the floured countertop. Quickly and carefully fold the edges of the dough into the center, flip the loaf so the seams are on the bottom and place the loaf onto the prepared board. Cover the loaf without touching the dough (a large inverted bowl or Tupperware container works well). Allow the dough to rise for a final 45 minutes.
With about 20 minutes to go in the last rise, put a 5-quart cast-iron Dutch oven with lid into the oven and set it to 500 F.
At the end of the final rising period, the oven and cast-iron pot should be fully preheated. Gather your two dough scrapers and a sheet of aluminum foil large enough to cover the top of the Dutch oven. Remove the pot from the oven.
With one dough scraper in each hand, carefully slide the scrapers under the loaf from either side and transfer the loaf to the cast-iron pot. Cover the pot with the foil and place the preheated lid on top. Return the pot to the oven. Bake for 12 minutes, then remove the lid and foil. Bake until the surface of the loaf is a deep brown, another 12 to 15 minutes.
Remove the loaf to a cooling rack. Allow it to fully cool before slicing.
Serve and enjoy!