How to Make It
Place bread flour, whole-wheat flours, yeast, and 31/2 cups warm water in a large bowl. Using your hand as a claw, stir until dough forms a shaggy mass and no bits of dry flour remain. Cover bowl with a towel; let stand 20 minutes.
Using wet hands, stretch and fold dough over itself in bowl 3 times, rotating bowl a third turn with each fold. Cover; let stand 15 minutes. Repeat process 3 times, totaling 4 folding cycles over 1 hour.
Lightly coat a 10- x 8-inch food-safe rectangular plastic bin or large, deep turkey roasting pan with cooking spray. Transfer dough to bin. Cover with plastic wrap; chill 8 hours or overnight.
Preheat oven to 550°F with a baking stone in lower third position of oven. Turn dough out onto a heavily floured surface, maintaining rectangular shape of dough and handling as little as possible. Dust top of dough generously with flour; cut into 4 equal rectangles (about 5 x 4 inches each) using a bench scraper. Generously flour a large linen kitchen towel (not terry cloth). Gently stretch each dough rectangle by hand into a roughly 12- x 5-inch rectangle. Place stretched dough rectangles on towel, pulling up small folds of cloth between each rectangle to keep dough separated. Cover dough rectangles with outer edges of towel or with a separate floured towel if necessary; let dough proof until slightly increased in volume, 45 minutes to 1 hour.
Gently invert 2 dough rectangles onto an unrimmed baking sheet lined with parchment paper. Slide parchment paper with dough onto preheated baking stone in oven. Reduce oven temperature to 500°F; bake until loaves are golden brown and a thermometer inserted in center of loaves registers 205°F, 16 to 20 minutes. Transfer baked loaves to a wire rack. Increase oven temperature to 550°F. Once temperature has returned to 550°F, repeat process with remaining 2 dough rectangles. Let loaves cool completely on wire rack; slice and serve.