Preheat the oven to 350°F (175°C).

Wash and slice the eggplant, zucchini, yellow squash, and onion into thin rounds (⅛" to ¼" thick).

In a mixing bowl, combine the can of tomato sauce with Italian seasoning, chopped sweet basil, and a pinch of salt. Stir until well blended.

Pour the seasoned tomato sauce into the baking dish, then layer the sliced vegetables over the sauce in a circular pattern.

Drizzle olive oil over the top and season with salt and freshly cracked black pepper to taste.

Bake for 55 to 60 minutes until the vegetables are tender and the sauce is bubbling.

Remove from the oven and sprinkle with freshly grated Parmesan if desired. Let cool slightly before serving.