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.