Lightly spoon the flour into dry measuring cups, and level with a knife. Dissolve the sugar and the yeast in 1 1/4 cups warm water in a large bowl. Stir in 1 cup flour. Cover the mixture loosely with plastic wrap, and let stand 30 minutes.
Add 2 cups flour, white cornmeal, 1 teaspoon salt, chopped rosemary, and 1 1/2 teaspoons oil to yeast mixture; stir until a soft dough forms.
Turn dough out onto a floured surface. Knead until smooth and elastic (about 8 minutes); add enough of remaining flour, 1 teaspoon at a time, to prevent dough from sticking to hands (dough will feel tacky).
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 hour or until doubled in size. (Press two fingers into dough. If indentation remains, the dough has risen enough.)
Punch dough down, and turn out onto a lightly floured surface. Arrange 2/3 cup grapes over dough, and knead gently 4 to 5 times or just until grapes are incorporated into the dough. Let rest 5 minutes.
Press the dough into a 15 x 10-inch rectangle. Place on a large baking sheet coated with cooking spray. Brush the surface of the dough with 1 teaspoon oil. Cover and let rise 30 minutes or until doubled in size.
Preheat oven to 475°.
Uncover dough. Make indentations in top of dough using the handle of a wooden spoon or your fingertips. Sprinkle surface of dough with remaining 1/3 cup grapes; gently press grapes into dough. Sprinkle with 1/4 teaspoon salt. Bake at 475° for 20 minutes or until golden. Garnish with rosemary sprig, if desired.