At 8 o'clock in the evening, mix up the poolish. Place 250 grams of the white flour and all of the yeast in a bowl and stir.
Add 250 grams of water at 80° F. Mix well and cover.
Let rest overnight.
At 10 o'clock the next morning, place the remaining 100 grams of white flour, all of the whole wheat flour, and all of the salt in a bowl and stir.
Pour 150 grams of water at 105° F around the edge of the poolish and then tip everything into your new flour mixture.
Mix by hand. Cut the dough in pieces with your fingers, and then stretch and fold the dough over itself several times. Repeat until well mixed.
Let rest for half an hour, and then repeat the stretching and folding process several times, until the dough tightens slightly.
Let rest for 20 minutes, fold, let rest 20 minutes again, and then fold one last time.
Around 4 o'clock in the evening, when the dough is about 2½ times its original volume, it is ready to be divided. Carefully tip it out onto a floured surface, and dust with more flour. Using a sharp knife, cut the dough into two equal pieces.
Stretch and fold one of the dough balls, and then turn it upside down onto a clean surface. Pull the dough toward you several times, using the friction of the sticky dough on the clean counter to tighten it again. Repeat with the other ball.
Gently place the dough balls on a floured baking pan, leaving room for them to grow. Cover loosely, and let rise at room tempurature for an hour, then in the refrigerator for an hour.