Heat oil in large saucepan. Add onions; cook until softened, about 3 minutes. Add garlic, peppers, zucchini, eggplant, tomato, wine, oregano, salt, pepper and bay leaf. Bring to boiling; cover and simmer 15 minutes. Uncover; cook 10 minutes more until liquid is reduced by half. Remove from heat; stir in parsley.
Heat broiler. Coat baking pan with nonstick cooking spray. Arrange polenta on pan.
Broil 4 inches from heat until polenta is lightly golden, about 3 minutes. Transfer to platter. Remove bay leaf from ratatouille; spoon over polenta.