In a bowl, combine the whole wheat flour and bread flour with the water and add the starter. Mix for 1 minute and then add the salt. Knead the dough with a stand mixer fitted with the dough hook on medium speed for about 7 minutes, or by hand for about 12 minutes, or until the dough is smooth and passes the windowpane test. Turn the mixer to high speed if needed to get the dough to slap against the sides of the bowl. Allow to rise, covered with plastic wrap or a moist towel, at room temperature for about 6 hours or in the refrigerator for about 15 hours, or until doubled in volume.
Form 2 loaves and put into 2 oiled 6-cup loaf pans or shape the loaf into a boule. Allow to proof, covered with plastic wrap at room temperature or uncovered in a proofing box for about 3 hours, or until doubled in size.
Preheat the oven to 500°F. Slide a sheet pan on the floor or bottom rack of the oven and allow to heat for 5 minutes. Pour in enough hot water to just cover the bottom of the sheet pan. Place the loaf in the oven above the water in the pan and immediately close the oven door. Crack open the door and spray water into the oven. Wait for 30 seconds, crack open the door, and spray again; repeat once more after 30 seconds. Turn down the oven to 425°F. Bake for about 45 minutes, or until the bread is golden brown and a thermometer stuck into the bottom reads at least 205°F. Let cool on a cake rack.

VARIATION
Sourdough whole wheat raisin breas:
Knead 1 cup coarsely chopped walnut halves and 1/2 cup raisins into the dough and allow to rise for about 4 hours, or until doubled in volume. Oil six 2-cup loaf pans. Punch down the dough, shape into 6 small loaves, and put into the prepared pans. Allow to proof for 1 to 4 hours, until almost doubled in volume. Bake at 450°F for about 25 minutes, or until golden brown. Let cool on a rack.