Make sure to feed the white sour starter 18 hours before mixing the dough.
For the dough: Add the white sour starter, all of the flours, and the water to the mixer bowl. Mix on low speed for 3 minutes, and then let rest for 15 minutes covered with plastic. Add the salt and mix for 5 more minutes on low speed. The dough should achieve full gluten development; check the gluten development by performing a “window” test (see page 93).
Transfer the dough to a floured wooden work table and bulk ferment for 1 hour. Punch the dough down and fold it onto itself. Bulk ferment for 1 more hour.
Fold the dough onto itself one more time and bench rest for 15 minutes.
Divide the dough into forty 350-g/12.35-oz pieces.
Round the dough into a ball, pulling it toward you to tighten it. Transfer to heavily floured boule baskets (see Resources). The smooth top goes into the basket first; the seam should face upward. Bulk ferment at room temperature for 2 hours, covered with plastic wrap. Place in the refrigerator to rest and retard the yeast overnight.
Pull the dough out of the refrigerator 1 to 2 hours before you intend to grill it and check the proof. The dough should spring back when you apply gentle pressure with your fingertips.
Turn a grill on as high as it can go. Preheat a convection oven to 160°C/325°F.
Brush water over the surface of the dough while it is in the basket.
Apply 2 sage, 3 thyme, and 1 rosemary stem to each loaf. Push gently into the dough.
Turn the baskets, one at a time, over on top of the grill. Gently ease the dough out of the basket. Once you have pronounced grill marks on one side, after about 6 minutes, flip the dough over and grill to obtain grill marks on that side as well, 5 to 6 more minutes.
Check the internal temperature of the dough. It should read 95°C/205°F. If it has not reached this temperature and it already has pronounced grill marks, place the loaf in the preheated convection oven to finish cooking.
Cool on a wire rack or metro shelf. Reserve at room temperature. Serve at room temperature. One loaf will serve 4 to 6 people.