Heat the oven to 425°F and grease a 9-by-13-inch baking dish.
Toss the zucchini, bell pepper and mushrooms with the olive oil, Italian seasoning, salt and pepper on a sheet pan.
Roast the vegetables for 20 minutes, stirring once, until browned and no liquid pools on the pan.
Spread 1 cup marinara in the baking dish and layer half the polenta slices, half the vegetables, 3/4 cup mozzarella and 1 tablespoon Parmesan.
Repeat the polenta, vegetable and cheese layers and finish with the remaining marinara, mozzarella and Parmesan.
Lower the oven to 400°F and bake for 25 to 30 minutes, until the sauce bubbles and the top browns.
Rest for 10 minutes before cutting into squares.