Split bread in half horizontally. Place halves on baking sheet, cut side up.
In a 10-inch skillet or wok heat the salad dressing. Add onion, zucchini, green pepper, and garlic. Stir-fry for 3 to 5 minutes or until crisp-tender. Stir in tomato. Sprinkle half of the provolone or mozzarella cheese on bread halves. Spoon vegetable mixture on cheese; sprinkle with remaining provolone or mozzarella and the Parmesan cheese.
Broil 3 to 4 inches from the heat about 2 minutes or until cheese melts. Serve immediately. Makes 4 servings.