Dissolve yeast in warm water in a large bowl; let stand 5 minutes. Lightly spoon flour into dry measuring cups; level with a knife. Add 2 cups flour and sugar to yeast mixture; stir until well combined. Cover and let stand at room temperature 1 hour to form a sponge (mixture will rise and bubble slightly).
Add 1 cup flour, 1/2 cup cornmeal, 1 tablespoon olive oil, 1/4 teaspoon black cumin seeds, and 1 teaspoon salt to sponge; stir until a soft dough forms. Turn dough out onto a floured surface. Knead until smooth and elastic (about 8 minutes); add enough of the 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 hour or until doubled in size. (Gently press two fingers into dough. If indentation remains, dough has risen enough.) Punch dough down; cover and let rest 5 minutes. Turn dough out onto a lightly floured surface; gently knead in 2/3 cup grape halves. Cover dough with plastic wrap, and let rest for 10 minutes.
Transfer dough onto a baking sheet sprinkled with remaining 1 teaspoon cornmeal. Pat dough into a 14 x 10-inch rectangle; brush with remaining 1 tablespoon olive oil. Cover and let rise 30 minutes or until doubled in size. (Gently press two fingers into dough. If indentation remains, dough has risen enough.)
Preheat oven to 475°.
Uncover dough, and sprinkle with remaining 1/2 teaspoon cumin seeds and kosher salt. Sprinkle remaining 1/3 cup grape halves over dough; gently press into dough.
Bake at 475° for 15 minutes or until lightly browned. Loosen bread from pan with a thin spatula. Serve warm or at room temperature.