Heat oven to 375ºF. Spray 12-inch nonstick skillet with cooking spray; heat over medium-high heat. Cook onion and bell pepper in skillet 2 minutes, stirring occasionally. Stir in eggplant, zucchini, salt and pepper. Cook 3 to 4 minutes, stirring occasionally, until vegetables are tender. Stir in tomatoes, breaking up with spoon; reduce heat to low. Cook 3 minutes, stirring occasionally.
Spray rectangular baking dish, 11x7x1 1/2 inches, with cooking spray. Cut polenta into 1/4-inch slices. Arrange slices on bottom of dish, overlapping and cutting to fit where necessary. Sprinkle with Parmesan cheese. Spoon vegetable mixture evenly over top.
Cover and bake 30 minutes. Sprinkle with mozzarella cheese and parsley. Bake uncovered about 15 minutes or until cheese is melted and casserole is bubbly. Let stand 5 minutes before serving.