This dough needs time to develop full flavor, but the result is a bakery-quality baguette made in your own kitchen. Thanks to a steam-heated baking technique, this baguette has a complex outer crunch paired with a soft interior. (For baking, accurate metric measurements produce the best results, so we recommend using a digital scale for this recipe.)
Starter: Also called pre-ferment, levain, or poolish (which contains commercial yeast, as in this starter), this is the leavening or rising component of the bread. 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. Add water to the container, then the pinch of yeast. Tare to zero again and add the correct amount of flour to the water and yeast. Use a chopstick to stir the starter until the dry and wet ingredients are just mixed. 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 and use a rubber spatula loosen the starter. Empty contents into a bowl and use a sturdier wooden spoon to mix, so there are fewer lumps. Stir in pinch of yeast. Then add white flour, rye 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 look shaggy, with some flour still visible in the bowl.
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. If there are any dry bits, pile them on the center of the dough so they get absorbed. Use the scraper to stretch the dough into a rectangle (dough will still be slightly shaggy); 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, and continuing to pile any loose bits of dough onto the center to be absorbed. (When the dough feels firm enough and is no longer sticky, set the scraper aside and use only your hands.) Continue rolling and tucking the dough until the dough feels stronger and begins to resist any further rolling, about 10 times. All of the flour will get absorbed into the dough during this process. Shape the dough into a ball and place into a bowl coated with nonstick spray. Cover 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 done twice, with a resting period after each stretch and fold. Combine 1 part fine semolina with 5 parts unbleached white flour to make dusting flour, and lightly dust the work surface and your hands. Release the dough from the bowl (being gentle to avoid tearing!) and place on the work surface. Flip back and forth (like pizza dough), and gently shape it into a rough rectangle. Fold the dough in thirds, like a letter. Pat it down slightly, then do 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 and final stretch and fold, repeat the steps for the first stretch and fold. Cover with plastic wrap and let rest at room temperature, 20 minutes.
Shaping/Fermentation (Proofing): Shape the dough to prepare it for baking, then ferment (proof) it in the refrigerator to expand the dough and develop flavor. Using a bench scraper, divide the dough into 2 equal pieces; it’s a good idea to weigh them so they’re the same size. Lightly dust your hands with dusting flour and roll each ball into a 6-inch log. Flatten slightly to press out some of the gas. Fold the top edge horizontally to meet the bottom edge, similar to the roll and tuck method in step 3. Press against the seam and roll several times until the dough is about 12 inches long, using a bit more pressure on the ends to taper them. Line a rimmed baking sheet with a cloth towel and dust with dusting flour. Arrange baguettes seam side up on the towel-lined pan in 2 rows. Fold the towel to create support ridges along the length of the baguettes. Lightly cover with another towel, and place in refrigerator to ferment, 12-16 hours.
Preheating Oven/Baking: Heat the cast-iron skillet while the oven is preheating. By filling the skillet with ice just prior to baking, you will create an optimal environment to steam the bread, resulting in a crispy crust and tender interior. Place cast-iron skillet on the bottom of a cold oven and place baking stone on lowest rack. Preheat oven to 460 F (preferably convection) for 1 hour. Remove baguettes from refrigerator and and test dough for baking by gently pressing; it should bounce back, leaving a slight dimple. Gently lift and roll baguettes (one at a time) from the linen liner onto the transfer peel. Then, transfer to the baking peel. Using a lame or razor blade, score the top of each baguette; this will create a controlled place for the gas to escape, thus preventing blow-outs. Make about 9 scores, at an angle, on top of each baguette, evenly spaced in the middle with a finger space on either side. Place baguettes directly on the preheated baking stone in the oven. Carefully fill skillet with about 2 cups of ice cubes. Close oven door and lower temperature to 440 degrees F. Bake until tops are lightly browned, 18–20 minutes.
Cooling, Serving, and Storing: Using the baking peel, remove baguettes from the oven and cool completely, about an hour. Baguettes are best eaten the day they’re baked, but can be stored in a paper bag for up to 24 hours.