Prepare the Tomato Sauce. Heat olive oil in a pan over medium heat. Add the chopped onion and cook for 3-4 minutes until softened. Stir in the minced garlic and cook for 1 more minute. Add the crushed tomatoes, balsamic vinegar, oregano, basil, salt, and black pepper. Let simmer for 10 minutes, then spread the sauce evenly in the bottom of a baking dish.
Slice & Arrange the Vegetables. Using a sharp knife or mandoline, thinly slice the zucchini, squash, eggplant, and tomatoes into uniform rounds. Start arranging the slices in an alternating pattern (zucchini, eggplant, tomato, etc.) in a spiral or neat rows over the sauce.
Bake to Perfection. Drizzle a little olive oil over the vegetables and sprinkle with a pinch of salt and pepper. Cover the dish with foil and bake in a 375°F oven for 30 minutes. Remove the foil and bake for another 15-20 minutes until the vegetables are tender and slightly caramelized.
Garnish & Serve. Sprinkle fresh basil or parsley on top before serving. For an extra flavor boost, add grated Parmesan or crumbled feta before serving.