
In a mixing bowl or large bowl, combine starter, water and 4 cups of flour. Mix till smooth. Cover with plastic wrap and let rise till spongy, about 8 to 10 hours, or overnight.With a wooden spoon beat warm mashed potatoes with milk. Add sugar, salt and rosemary. Add remaining flour, one cup at a time, till dough is stiff to the touch. Place dough on a lightly floured surface and knead for 8 to 10 minutes. Place dough into a large oiled bowl. Turning it over to coat all sides. Cover with plastic wrap and let rise till double in size, about 2 hours. Punch down dough and place on a lightly floured surface once again. Divide the dough into 3 equal parts. Form each dough into a round loaf and place on a greased cookie sheet. Cover with plastic wrap till doubled in size, about 2 hours. Preheat over to 375 F. With a razor blade or very sharp knife, slash an X on the tops of each bread loaf. Mix the egg and remaining water ant brush the tops of each loaf. Place in oven. Take a pan of boiling water and place it on the bottom rack. Bake for 30 to 35 minutes or until golden brown. Cool in the pans on a wire rack.