Heat the oven to 350 F. Lightly coat a 3-quart ovenproof dish with cooking spray.
Combine the polenta, water and garlic in the prepared dish. Bake uncovered until the polenta pulls away from the sides of the baking dish, about 40 minutes. The polenta should be moist.
While the polenta is cooking, spray a nonstick frying pan with cooking spray. Add the mushrooms and onions. Saute over medium heat until the vegetables are tender, about 5 minutes.
In a pot fitted with a steamer basket, bring 1 inch of water to a boil. Add the broccoli and zucchini. Cover and steam until tender-crisp, 2 to 3 minutes.
When the polenta is done, top with the cooked vegetables. Sprinkle with Parmesan cheese and herbs, to taste. Serve immediately.