Place potato in a small saucepan and cover with water; bring to a boil. Reduce heat to medium-low, cover and cook until tender, 15 to 20 minutes. Drain, reserving 1 cup cooking liquid. Place the potato in a bowl and mash with a fork or potato masher. Drizzle with oil. Let mashed potato and reserved cooking liquid cool to lukewarm.

Dissolve sugar in water in a large bowl. Stir in yeast and let stand until foamy, about 5 minutes. Add the mashed potato and reserved cooking liquid. Gradually beat in whole-wheat flour. Beat for 1 minute. Stir in rosemary, thyme, sage and salt. Gradually beat in enough of the all-purpose flour until the dough is too stiff to beat. Turn the dough out onto a lightly floured surface. Knead until smooth and elastic, adding just enough flour to prevent sticking, about 10 minutes. (Alternatively, use a stand-up mixer fitted with a dough hook to mix and knead the dough.)

Place the dough in a lightly oiled bowl. Turn to coat and cover with plastic wrap. Let rise until doubled in bulk, about 1 1/2 hours.

Coat a large baking sheet with cooking spray. Sprinkle with cornmeal. Punch the dough down. Turn out onto the work surface and knead several times. Divide dough in half and shape each piece into a ball. Place loaves several inches apart on the baking sheet. Cover with plastic wrap and let rise for 1 hour.

Half an hour before baking, place a baking stone or inverted baking sheet on the middle rack of the oven. Place a small baking pan on the rack below. Preheat oven to 450 degrees F.

To decorate loaves: Place a large bowl of cold water beside the stove. Bring a large saucepan of water to a boil. Drop chives, parsley sprigs and sage leaves into the boiling water for a few seconds. Retrieve with tongs or a slotted spoon and drop into the cold water. Pat herbs dry.

Blend egg white and water with a fork in a small bowl; brush over the risen loaves. Arrange herb sprigs decoratively over the loaves. Brush again with the egg-white glaze.

Pour 1 cup water into the baking pan in the oven. Place the baking sheet on the baking stone (or inverted baking sheet) and bake the loaves for 20 minutes. Reduce oven temperature to 400 degrees and bake until the loaves are golden and the bottoms sound hollow when tapped, 10 to 15 minutes. Transfer to a wire rack and let cool.