1.Preheat oven to 220°C/200°C fan-forced.
2.Combine zucchini, eggplant, tomato, mushrooms, oil and garlic, in single layer, in large shallow baking dish. Roast, uncovered, about 20 minutes or until vegetables are tender.
3.Meanwhile, in a large saucepan, combine 2 cups of the milk and the water; bring to a boil. Gradually add polenta to liquid, stirring constantly. Reduce heat; simmer, stirring, about 5 minutes or until polenta thickens. Stir in cheese, spinach and remaining milk.
4.Divide vegetables among serving plates; top with polenta. Sprinkle with extra parmesan, if desired.