To make the bread, cook the potatoes and drain, reserving 1 cup of the cooking water. Mash the potatoes. In a large bowl, dissolve the yeast in the warm potato water (105° to 120°F). When bubbles start to form, blend in the mashed potatoes and rye sour. Add 3 cups of the all-purpose flour, salt, and caraway seeds. Mix with a wooden spoon until the dough pulls away from the sides of the bowl, adding up to 3 more cups flour as needed. Turn the dough out onto a floured surface and knead for about 5 to 8 minutes. Knead in the filling and enough flour to make a soft, slightly sticky dough. Place the dough on an oiled surface, cover, and let rest for 15 minutes.

Preheat the oven to 200°F. Divide the dough into 3 parts. Either shape into round loaves and place on baking sheets sprinkled with cornmeal or place in loaf pans coated with vegetable cooking spray. Place in the oven, turn the temperature off, and leave the door open while the loaves rise for about 45 minutes. Remove from the oven, then preheat the oven to 375°F. Place an empty baking pan (about 10x8 inches) on the lower rack of the oven while it is heating. Meanwhile, in a small bowl, combine the cold water and cornstarch. Add the boiling water and stir. With a pastry brush, coat the dough with the cornstarch mixture. Poke about 12 holes in each loaf with a skewer or peeled stick. Put about a dozen ice cubes in the baking pan and bake the loaves for 35 to 40 minutes, or until they sound hollow when tapped. Turn out onto wire racks, brush again with the cornstarch solution, and cool.