Step 1

Place white bread flour, whole wheat bread flour, and yeast into a large mixing bowl and stir with a wooden spoon. Pour in cold water and mix with a wooden spoon or spatula until a very wet, sticky dough comes together, about 3 minutes. Add the salt and mix another 2 to 3 minutes. Scrape down the sides of the bowl. Cover with foil. Allow dough to rise at room temperature for 18 hours.

Step 2

Deflate the bubbly dough with a spatula by scraping down the sides of the bowl and folding the dough over itself, turning the bowl as you go around in a circular motion, about 12 times or so.

Step 3

Scrape the dough onto a very well-floured surface with a spatula. Sprinkle the surface of dough generously with flour. Generously flour hands and then roll and fold dough on the table until you've formed a round or oval loaf shape that has a smooth surface, 1 to 2 minutes. The dough is very sticky, so add more flour if you need. The shape doesn't really matter, but a smooth surface is the goal.

Step 4

Transfer to a Silpat®-lined baking sheet and let rise, uncovered, until doubled in size, about 2 hours. The dough will spread out more than rise up, but this is normal.

Step 5

Preheat the oven to 450 degrees F (230 degrees C).

Step 6

Make 1 shallow slash down the middle of the dough with a very sharp knife or razor, being careful not to deflate the dough too much; the slash is optional. Spray the surface of the loaf very lightly with plain water to help the crust form.

Step 7

Bake in the center of the preheated oven until nicely browned, about 30 minutes. Let cool on a rack before cutting.