Add bread flour, whole-wheat flour, 1 1/2 cups water, oregano leaves, salt, and yeast to bread machine according to manufacturer's directions. Select dough cycle. At end of cycle, scrape dough onto a lightly floured board. Knead in leeks and walnuts.
Scrape dough onto a well-floured board and knead briefly to expel air. With lightly floured hands, gather 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 loaf on a well-floured surface, dust lightly with flour, cover loosely with plastic wrap, and let rise at room temperature until it's slightly puffy and holds 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 loaf to sheet. With a sharp knife, make three diagonal slashes 1 inch deep and 1 to 2 inches apart across loaf top. 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 loaf and lift it onto end of sheet. Slash as directed above, then gently slide loaf onto one side of stone in oven.
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 loaf to a rack to cool for at least 1 hour. Store in a paper bag at room temperature up to 2 days. To recrisp the crust, place loaf directly on a rack in a 400° oven and bake for about 5 minutes.
Nutritional analysis per ounce.