Combine 2 cups water, sugar, 2 teaspoons salt, and yeast in a large bowl, stirring with a whisk. Let stand for 5 minutes. Lightly spoon flour into dry measuring cups, and level with a knife. Add 5 1/2 cups flour, about 1 cup at a time, to the yeast mixture, stirring until a soft dough forms. Turn dough out onto a floured surface. Knead until dough is smooth and elastic (about 8 minutes); add enough of remaining flour, 1 tablespoon at a time, to prevent dough from sticking to hands.
Place dough in a large bowl coated with cooking spray, turning to coat top. Cover and let rise in a warm place (85°), free from drafts, 1 1/2 hours or until doubled in size. (Gently press 2 fingers into dough. If indentation remains, dough has risen enough.)
Divide the dough into 2 equal portions, shaping each portion into a 12-inch-long loaf. Place the loaves on a baking sheet sprinkled with 1/2 cup cornmeal. Lightly brush the loaves with cold water. Sprinkle evenly with 1/4 cup of sesame seeds.
Place a shallow pan of boiling water on the bottom rack of cold oven. Place loaves on middle rack of oven. Set oven temperature to 400°. (Do not preheat oven.) Bake for 35 minutes or until loaves sound hollow when tapped. Cool on a wire rack.