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Preheat the oven to 350ºF and grease a square baking dish with cooking spray or oil.
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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.
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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.
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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.
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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.