MIX cornmeal, milk, broth and 1/4 cup cheese in large microwaveable bowl with whisk until well blended. Microwave on HIGH 16 min. or until thickened, stirring well after 8 min.
MEANWHILE, cook zucchini, peppers, onions, tomatoes and dressing mix in large saucepan on medium heat 10 min. or until vegetables are crisp-tender, stirring frequently.
SPOON vegetable mixture over polenta; sprinkle with remaining cheese.