In a mixing bowl, combine 2 cups flour, yeast and salt. Add water and oil; beat until smooth. Stir in enough remaining flour to form a soft dough. Turn onto a floured surface; knead until smooth and elastic, about 4 minutes. Cover and let rest for 15 minutes. Meanwhile, in a bowl, combine the tomatoes, mushrooms, green pepper, olives, onion, oil, vinegar and seasonings.
Coat a 15-in. x 10-in.x 1-in. baking pan with nonstick cooking spray; sprinkle with cornmeal. Press dough into pan. Prick dough generously with a fork. Bake at 475 degrees F for 5 minutes or until lightly browned. Cover with vegetable mixture. Bake 8-10 minutes longer or until edges of crust are golden.