Step 2

Make the poolish: In a large bowl, stir together the water, yeast and bread flour until completely combined. Cover and let ferment until liquidy and bubbles have formed on the surface, 12 hours.

Step 3

Make the dough: Two hours before the poolish is ready, in a large bowl, combine the water, bread flour and whole wheat flour, and mix until a shaggy dough forms. Cover with plastic wrap and let sit, allowing the flours to hydrate, 1½ to 2 hours.

Step 4

Meanwhile, make the oatmeal porridge: In a medium saucepan, combine the oats and water, and bring to a simmer over medium-high heat. Reduce the heat to medium and cook, stirring frequently, until the oats are completely cooked and very soft, adding 1 to 2 tablespoons of water as needed until a thick porridge forms, 7 minutes. Remove from the heat and transfer to a medium bowl; season with salt and set aside to let cool completely.

Step 5

After the 12-hour resting period, the poolish should be bubbly and ready to be mixed into the dough. Add 120 grams (about 3/4 cup) of poolish to the dough, mixing with your hands until fully incorporated.

Step 6

Knead the dough: Lift the dough with both hands, stretching it a few inches above the bowl, and slap it back down in the bowl, and fold the dough on itself to create a bundle. Repeat this process until gluten development begins to form, 3 minutes, then add the salt and knead until long rope-like strands of dough are stretching over the surface, 4 to 6 minutes more.

Step 7

Add the oatmeal porridge to the dough and mix until just dispersed throughout (don't overmix or the oats will cut the gluten development). Place the dough in a large clear container (such as a 6-quart Cambro), cover and let sit 1 hour to ferment, until bubbles begin to form on the bottom and sides of the container.

Step 8

Do 2 stretch and folds: Using both hands, grab one end of the dough inside the container, stretch it into the air and fold it about halfway over itself. Do this on all four sides and let rest 1 hour. Do a second stretch and fold and let rest until bubbles form on the sides of the dough in the container, 1 to 2 hours more.

Step 9

Do the first dough shaping: Once the dough is ready, gently overturn the container and let the dough fall onto a lightly floured kitchen surface. Lightly flour a metal bench scraper and, with the scraper in your dominant hand and with the help of your free hand to guide, barely pick up the far end of the dough, dragging it away and back toward you while simultaneously rotating the dough counterclockwise, until a tight ball forms, 3 to 6 times. (Be careful not to pop the gas bubbles.) Let the dough rest on the kitchen surface 30 to 40 minutes.

Step 10

Do the second dough shaping: Lightly flour the kitchen surface next to the dough and gently flip the dough onto it. Flour the dough and stretch into a rectangle, with the long end in front of you, and fold the right side toward the center over a third of the dough, and the left side over that, as in forming a letter. Stretch the top down toward the bottom, making a square. From the top two corners down, grab the opposite ends and bring them together, as if sewing up the dough. Repeat all the way to the bottom of the dough, making 5 sews. When all sewn up, pull the top of the dough halfway down over itself and do 2 to 3 mores sews. Flip the top half of the dough down over the bottom half, with the smooth side on the top and the seam at the bottom.

Step 11

Line a large colander, wooden bowl or proofing basket with a clean kitchen towel and generously dust with rice flour. With the help of the bench scraper, gently lift and flip the dough into the floured basket, seam-side up. Let sit and proof at room temperature for 30 minutes, then refrigerate 12 to 16 hours. (This can vary depending on your schedule and when it is most convenient to bake.)

Step 12

Bake the bread: Place an empty large Dutch oven in the oven and preheat it to 500°. Let sit in the preheated oven until the Dutch oven is very hot, 30 minutes.

Step 13

When the Dutch oven is ready, use oven mitts to carefully remove it and place it onto a heatproof surface. Flip the dough from the basket into the Dutch oven, so the seam side is down. Using a sharp knife, score two 3-inch-long and ¼-inch-deep parallel incisions on the top of the dough. Place the lid on the Dutch oven and return it to the preheated oven. Immediately lower the temperature to 475° and bake for 15 minutes. Remove the Dutch oven lid and continue to bake until the crust is a rich, deep brown color, rotating halfway through baking, 30 to 35 minutes. Remove from the oven, and transfer the bread to a resting rack and let cool before slicing, 1 hour.