Instructions

For the Levain: Combine flour, water, and starter in bowl and stir with wooden spoon until uniform. Cover with plastic wrap and let sit at room temperature until tripled in volume, 5 to 12 hours. Use immediately or transfer to refrigerator and use within 24 hours.
For the Dough: Sift whole-wheat flour through fine-mesh strainer into large bowl; discard bran remaining in strainer. Add all-purpose flour, water, and 1 cup (8 ounces; 227 grams) levain (discard remainder) and stir with wooden spoon until cohesive dough forms and no dry flour remains. Cover with plastic wrap and let rest at room temperature for 20 minutes. Sprinkle salt over dough and knead gently in bowl until incorporated. Cover with plastic wrap and let rest at room temperature for 30 minutes.
Holding edge of dough with your fingertips, fold dough over itself by gently lifting and folding edge of dough toward center. Turn bowl 45 degrees; fold again. Turn bowl and fold dough 6 more times (total of 8 folds). Cover with plastic and let rise for 30 minutes. Repeat folding and rising every 30 minutes, 3 more times. After fourth set of folds, transfer dough to lightly floured counter.
Gently press dough into 8-inch disk, then fold edges toward middle to form round. Cover loosely with plastic and let rest for 15 minutes. Meanwhile, line colander with large linen or cotton dish towel and dust liberally with flour. Repeat pressing and folding of dough to form round, then place dough seam side down on counter and form into tight round. (To round, set dough on unfloured counter. Loosely cup your hands around dough and, without applying pressure to dough, move your hands in small circular motions. Tackiness of dough against counter and circular motion should work dough into smooth, even ball, but if dough sticks to your hands, lightly dust your fingers with flour.)
Place dough seam side up on floured towel and loosely fold edges of towel over dough to enclose. Place colander in large plastic garbage bag and tie or fold under to fully enclose. Let rest at room temperature for 1 hour, then refrigerate for 12 to 24 hours.
Adjust oven rack to middle position and place loaf pan or cake pan in bottom of oven. Remove colander from refrigerator and place on middle rack; pour 3 cups boiling water into pan below. Close oven door and let dough rise until doubled in size and does not readily spring back when poked with your finger, 2 to 3 hours.
Remove colander and water pan from oven. Lay 12 by 12-inch sheet of parchment paper on counter and spray generously with vegetable oil spray. Remove colander from plastic bag, unfold edges of towel, and dust top of loaf with flour. Lay parchment sprayed side down over loaf, then invert colander onto counter. Remove colander and towel. Holding razor blade or sharp knife at 30-degree angle to loaf, make series of four 4-inch long, 1/2-inch deep slashes to form square around perimeter of loaf, about 2 inches in from edge of loaf. Pick up dough by lifting parchment edges and lower into heavy-bottomed Dutch oven. Cover pot and place in oven. Heat oven to 425 degrees. Bake bread for 30 minutes (starting timing as soon as you turn on oven).
Remove lid and continue to bake until bread is deep brown and registers 210 degrees, 20 to 30 minutes longer. Carefully remove bread from pot; transfer to wire rack and let cool completely, at least 2 hours.