Don’t skip this recipe because of the 30-hour moniker; the majority of it is resting time to develop the crumb and the flavor, and it's worth it. You’ll need sourdough starter, an essential ingredient, before you begin. With a little patience, this will become your go-to bread recipe. (For baking, accurate metric measurements produce the best results, so we recommend using a digital scale for this recipe.)
Starter: Also called pre-ferment or levain, this is the leavening or rising component of the bread baking process. Turn on a digital scale and weigh your empty container. Deduct the weight of the container by pressing the "tare" function, which resets the scale to zero. Prepare the stiff "mother" sourdough starter for the dough by measuring 50 grams into the container, reserving remaining starter for future use (see Sourdough Starter recipe). Tare the scale to zero again, then add water. Break the starter into pieces with your fingers until it’s almost dissolved; then add the flour and stir with a chopstick until it's incorporated. Cover the container with plastic wrap and let rest at room temperature, away from direct sunlight, for 10-16 hours, but optimally 12 hours.
Dough: Place container with starter onto the scale and tare to zero; then add water around the edges of the container to loosen the starter. Empty contents into a bowl and stir so there are minimal lumps. Add white flour, wheat flour, light rye flour, dark rye flour, buckwheat flour, and salt. Using a wooden spoon, and starting in the middle of the bowl, slowly stir to the edges until most of the flour has been mixed in and hydrated. Dough will be stiff and sticky.
Roll and Tuck: A technique to develop gluten in the dough so it becomes firmer and easier to work with. This is done one time, followed by a resting period. Using a plastic bowl scraper, push the dough and residual flour out of the bowl onto a clean work surface. Combine 1 part fine semolina with 5 parts unbleached white flour to make dusting flour, and dust the surface around the dough. Use the scraper to stretch the dough into a loose rectangle (dough will be sticky); then, starting at one end, roll and tuck the dough like a tube, about 3-4 times around. Flip the dough so it’s seam-side up; then flatten again to a rectangle so that the seam runs from left to right. Repeat rolling and tucking, always returning to a seam-up position. (When the dough feels firm enough and is no longer sticky, set the scraper aside and use only your hands.) Using dusting flour as needed, continue rolling and tucking the dough until it feels stronger and begins to resist any further rolling, about 12 times. Shape into a ball and place into a bowl coated with nonstick spray. Cover the bowl with plastic wrap and let rest at room temperature, 45 minutes.
Stretch and Fold: This technique strengthens the dough by folding it over onto itself. This process is repeated 3 times, with a resting period after each stretch and fold. Lightly dust the work surface and your hands with dusting flour. Using the scraper, release the dough from the bowl and place it seam-side down on the work surface. Gently stretch the dough into a rectangle (using flat "mitten" hands, rather than "talons"), then fold the dough into thirds, like a letter. Pat it down slightly, then repeat the same fold in the other direction. Place the dough back in the bowl, cover with plastic wrap and let rest at room temperature, 45 minutes. For the second stretch and fold, repeat the steps for the first stretch and fold. Cover with plastic wrap and rest at room temperature, 45 minutes. For the third and final stretch and fold, repeat the steps for the first and second stretch and folds, cover with plastic wrap and rest at room temperature, 30 minutes.
Shaping/Fermentation (Proofing): Shape the dough to prepare it for baking, then ferment (proof) it for a period of time to expand the dough and develop flavor. Line a 9-inch bowl with a cloth towel; coat the cloth generously with dusting flour. Lightly dust the work surface and your hands with dusting flour. Press the dough down slightly to de-gas it, then fold it in half. Bring the ends together and gently shape the dough into a ball, allowing the work surface to help you tighten the bottom of the loaf. (If you wish, you can "pinch" the seam shut.) Transfer dough to the prepared bowl, seam side up. Loosely fold the edges of the towel over the top, and let rest at room temperature, 1-1½ hours. Then place it in the refrigerator to ferment, 16 hours.
Preheating the Oven: Preheat the Dutch oven as your oven comes to temperature, because using a hot baking vessel will give you a crisper crust. In a cold oven, place baking stone on a rack. (This is optional, but keeping a baking stone in the oven during baking will ensure more even heat distribution.) Place Dutch oven on top of the baking stone. Preheat oven to 500 F (preferably convection). Meanwhile, remove dough from refrigerator and let it come to room temperature while the oven preheats, 1 hour.
Baking: Start baking in the Dutch oven with the lid on, so the loaf will rise, then with the lid off, so it can brown. Using heavy oven mitts (and a great deal of caution), remove preheated Dutch oven, then remove lid. Dust the dough with flour; then very gently remove dough from bowl and turn onto a baking peel, floured seam side down. Using a lame, score the top of the dough: this allows you to control exactly where the gas will escape as the loaf bakes, thus preventing blow-outs. Carefully transfer bread to the Dutch oven, scored side up. Replace the lid and return the Dutch oven to the oven. After 5 minutes, lower the temperature to 460 degrees F and continue baking, 20–25 minutes. Then remove the lid and continue baking uncovered, 10 more minutes, until the surface is a deep, rich brown, with certain areas along the scores being even slightly darker: this is bread that's bien cuit ("well done").
Cooling, Slicing, and Storing: Remove Dutch oven. Wedge a wooden spoon under the hot loaf to lift it out of the Dutch oven; transfer it to a cooling rack. Turn upside down and check for doneness by tapping the bottom of the loaf: it should sound like a hollow log. Turn the loaf right side up and cool completely before slicing, to allow gases and flavors to redistribute evenly throughout the loaf, 2 hours. (Chef Golper recommends resting the baked bread for 1 day to further develop the flavor.) Store in a paper bag or wrap in cloth and keep at room temperature for up to 5 days.