In the bowl of a standing mixer or another large bowl, sprinkle yeast over 1 cup warm (100° to 110°) water; let stand until foamy, about 10 minutes.
Add 1 1/4 cups cold water, biga, 3 cups bread flour, whole-wheat flour, and salt to yeast mixture. Beat with paddle attachment on low speed, or stir with a heavy spoon, until well blended. Gradually beat or stir in 2 more cups bread flour, 1/2 cup at a time, until mixture forms a soft dough.
Switch to a dough hook and beat on medium speed until dough is smooth and elastic and pulls cleanly from sides of bowl but is still slightly sticky, 6 to 8 minutes; or scrape dough onto a lightly floured board and knead by hand until smooth and elastic but still slightly sticky, 7 to 10 minutes.
Add leeks, nuts, and oregano and beat in with dough hook or knead in by hand just until incorporated (after mixing in by hand, place dough in a bowl).
Cover bowl with plastic wrap; let dough rise at room temperature until doubled, 2 to 2 1/2 hours. Punch down with your hand to expel air.
Re-cover dough with plastic wrap and let rise again until doubled, 1 to 1 1/2 hours. Or for a slow rise, chill at least 8 and up to 12 hours; let come to room temperature, about 3 hours.
Scrape dough onto a well-floured board and knead briefly to expel air. Divide in half. With lightly floured hands, gather each half into a ball, then stretch and tuck edges under to shape into a smooth oval with slightly tapered ends (about 8 in. long and 4 in. wide in the center). Place loaves on a well-floured surface, dust lightly with flour, cover loosely with plastic wrap, and let rise at room temperature until they're slightly puffy and hold the imprint of a finger when lightly pressed, about 1 1/2 hours.
Sprinkle a 13- by 17-inch baking sheet generously with cornmeal. Transfer loaves, one at a time, to sheet, spacing 2 to 3 inches apart. With a sharp knife, make three diagonal slashes 1 inch deep and 1 to 2 inches apart across loaf tops. Place sheet on rack in lower third of a 450° regular or convection oven.
Or, if using a baking stone, gently slide edge of cornmeal-covered baking sheet under one loaf and lift it onto end of sheet. Slash as directed above, then gently slide loaf onto one side of stone in oven, leaving room for second loaf. Repeat to slash and transfer second loaf.
Spray 3 to 4 squirts of water on floor or sides of oven, taking care not to spray near heating element or lightbulb, then quickly close door.
Bake bread, spraying twice more at 5-minute intervals during the first 10 minutes of baking, until crust is well browned, 35 to 45 minutes total.
Transfer loaves to a rack to cool for at least 1 hour. Store in paper bags at room temperature up to 2 days. To recrisp the crust, place loaves directly on a rack in a 400° oven and bake for about 5 minutes.
Biga: In a bowl, sprinkle 1/4 teaspoon active dry yeast over 1/4 cup warm (100° to 110°) water. Let stand until foamy, about 5 minutes. Add 1/2 cup cold water. With a wood spoon, stir in 1 1/2 cups bread flour until mixture forms a soft dough. Cover with plastic wrap and chill 12 to 24 hours. Let come to room temperature before using, about 1 hour.
Shortcut: Without the biga, our recipes still produce great loaves. In the basic recipe, just increase the yeast by 1 1/4 teaspoons, the bread flour by 1 1/2 cups, and the water by 3/4 cup.
Nutritional analysis per ounce.