In a large bowl dissolve the yeast in the warm water or potato water. Add sugar and salt and mix well. Stir in enough flour to pull the dough into a ball.
On a well-floured surface, knead dough for 7 minutes, incorporating in the remaining flour. Place dough in a greased bowl. Let rise until doubled.
Using a lightly floured board, form dough into 2 long or round loaves. Sprinkle corn meal on a greased baking tray; place loaves on top. Slash top with a sharp knife. Cover, and let rise until almost doubled.
Brush top of loaves with cold water. Sprinkle with sesame seeds, if desired. Place in a cold oven.
Turn oven to 200 degrees C/Gas Mark 6. Bake for 45 minutes, or until nicely browned. Brush twice during baking time with cold water.