Thinly slice vegetables so they have a consistent width. Consider using a mandolin to help with this step.
Cover bottom of 9×9 baking dish with sauce. Stir in garlic and most of the fresh herbs. Season with salt and pepper. Layer onions on top of sauce.
Arrange eggplant, zucchini, potatoes and tomatoes upright in alternating patterns in the pan.
Drizzle with olive oil, sprinkle with remaining herbs.
Cover with aluminum foil and bake 40 minutes. Remove foil, and sprinkle with Parmesan, if using. Return to oven and bake an additional 20 minutes. Serve hot.