To prepare dough, combine first 4 ingredients in a large bowl, stirring to dissolve yeast; let stand 5 minutes. Lightly spoon flour into dry measuring cups; level with a knife. Add 2 1/2 cups flour, cornmeal, and 1 teaspoon salt to yeast mixture; stir well to combine. Turn dough out onto a floured surface. Knead until smooth and elastic (about 10 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 hour and 30 minutes or until doubled in size. (Gently press two fingers into dough. If indentation remains, dough has risen enough.)
To prepare topping, heat 1 tablespoon oil in a large nonstick skillet over medium-high heat. Add onion, dried basil, 1/2 teaspoon salt, and pepper; cook 20 minutes or until onion is golden, stirring occasionally. Cool.
Arrange tomato slices between several layers of paper towels. Let stand 10 minutes, pressing down occasionally.
Uncover dough. Punch dough down; cover and let rest 5 minutes. Pat dough into an even layer in a 15 x 10-inch jelly-roll pan coated with cooking spray. Cover and let rise 30 minutes.
Preheat oven to 425°.
Uncover dough. Arrange onion mixture over dough, leaving a 1/2-inch border. Top with tomato slices; sprinkle with cheese. Bake at 425° for 23 minutes or until browned. Sprinkle with fresh basil. Cool at least 10 minutes before serving.