1 lb. potatoes (4 medium) peeled and coarsely chopped
1 cup buttermilk
1 tsp. sugar
1 tsp. active dry yeast
4 cups whole wheat flour
1 1/2 Tbsp. salt
5-6 cups all purpose flour or bread flour
Place potatoes in medium saucepan with 1 cup of water. Cover and bring to a boil. Reduce heat to low and cook, covered, until potatoes are soft, about 10 minutes. Transfer potatoes and cooking liquid to a blender or food processor and puree. You should have 3 cups of puree. (Add water if necessary to make 3 cups) Transfer the puree to a large bowl and stir in buttermilk; cool to lukewarm.
In a measuring cup or small bowl, stir together 1 cup lukewarm water and sugar. Sprinkle with yeast and stir until it has dissolved, about 5 minutes. Stir in potato mixture.
One cup at a time, mix in whole wheat flour. Stir 100 times in the same direction (about one minute) to properly develop the gluten. Cover and wrap with plastic wrap and let stand 30 minutes or up to 2 hours.
Uncover, sprinkle with salt and stir gently. One cup at a time, stir in white flour until the dough becomes too difficult to stir. Turn the dough onto a well floured surface and knead, gradually working in more flour as necessary to prevent sticking, until the dough becomes smooth and slightly elastic, about 10 minutes.
Place the dough in a large lightly oiled bowl (or spray with nonstick cooking spray) and turn to coat. Cover with plastic wrap. Let rise until doubled in volume, 2 to 3 hours.
Lightly oil a large baking sheet. Gently punch down the dough. Transfer to a lightly floured surface and cut into two equal pieces. Working with one piece at a time, knead the dough briefly then shape into a high round mound, about 8 inches in diameter. Repeat with the other piece of dough. Place each dough mound on the baking sheet, leaving a half inch between loaves and the rims, and 1 inch between loaves. Dust each loaf with flour. Cover with plastic wrap and let rise until almost doubled, about 30 minutes.
Meanwhile, preheat oven to 400*F. Position rack in the lower third of oven.
Using a thin sharp knife, slash the tops of the loaves in a criss cross pattern, making two 3/4 inch deep parallel slashes 2 inches apart in each direction. Use a spritzer to liberally mist the loaves with water just before placing in oven. Bake for 5 minutes. Reduce heat to 375*F and bake until loaves are golden and sound hollow when tapped on the bottom, another 35 to 40 minutes. Transfer the loaves to wire racks to cool. Cut each loaf into 16 slices.