Combine pizza crust mix and cornmeal; add 2/3 cup water, stirring well. Shape into a ball; cover and let stand 5 minutes. Pat dough into a 14- x 11-inch rectangle on a large baking sheet coated with cooking spray. Bake at 450° for 5 minutes.
Coat a large nonstick skillet with cooking spray; add oil. Place over medium-high heat until hot. Add onion and squash; cook, stirring constantly, 10 minutes or until vegetables are tender and onion is golden. Stir in herb-and-spice blend and salt.
Brush crust with dressing; top with onion mixture and tomato slices. Sprinkle with cheese. Bake at 450° for 10 minutes or until crust is golden and cheese melts. Serve immediately.