Preheat the oven to 350ºF and grease a square baking dish with cooking spray or oil.
Heat the oil in a large skillet over medium heat. Add the onion, garlic, a pinch of salt and a few twists of freshly ground black pepper. Cook, stirring occasionally, for about 5 minutes, or until the onions are softened and just beginning to turn golden.
Add the sliced mushrooms. Let the liquid cook out of the mushrooms, then continue to cook them until the liquid evaporates and the mushrooms are lightly browned, 5-7 minutes. Stir in the zucchini and cook for 3 minutes more, or until the zucchini is tender. Stir in the marinara, then season with salt and pepper to taste.
Place half of the polenta rounds in the baking dish. Top with half of the vegetable mixture, then half of the mozzarella. Repeat with the remaining ingredients.
Bake the casserole for 30 minutes, or until the cheese is melted and just beginning to brown. Let it sit for 5 minutes or so, then cut into 4 portions and serve with fresh basil and Parmesan, if desired.