Heat oven to 400°. Grease a 2-quart oval baking dish.
Around edge of dish, alternately fan tomato and zucchini slices. They will be standing up on edge once finished. In middle of dish, fan eggplant slices so that they overlap. Randomly tuck in onion slices and sprinkle garlic over top. Drizzle 4 tbsp of the olive oil over gratin and season with salt and pepper. Sprinkle marjoram leaves over top.
Cover with foil and bake at 400° for 60 minutes. Dollop ricotta over top and sprinkle with Asiago and panko. Bake, uncovered, for an additional 15 minutes or until vegetables are tender.
Cool 10 minutes. Drizzle with remaining 2 tbsp olive oil and serve.