Place white and wheat flour in a large bowl. Add salt, yeast, and water. Mix until a wet sticky dough comes together, about 5 minutes. Scrape down sides of bowl. Cover with foil. Allow dough to rise for 18 hours at room temperature. It should not be too warm.

Punch dough down with a spatula and fold it over a few times.

Lightly grease a heavy-rimmed baking sheet with olive oil. Sprinkle generously with cornmeal.

Lightly spray a work surface with water. Place a long sheet of plastic wrap on the damp surface to hold it in place. Sprinkle plastic wrap with flour. Scrape the dough onto the floured surface. Sprinkle flour on top of the dough. Gently stretch and pull dough into a long, flat rectangular shape, 12 to 15 inches long. Bring plastic sheet to edge of prepared pan and flip the dough into the prepared pan. Reshape the dough, if necessary. Dust with flour. Cover with a light dry towel. Let rise about 2 hours.

Preheat oven to 425 degrees F (220 degrees C).

Bake dough in preheated oven until loaf is nicely browned, 35 to 45 minutes.