1. In a bowl, combine oil, salt, garlic, oregano, rosemary, thyme, parsley, pepper, lemon zest and juice. Put zucchini, squash, bell peppers and onion in a large ziplock bag and pour in half of marinade. Seal bag and shake to coat vegetables. Let stand for 3 hours at room temperature. Put eggplant and mushrooms in a separate ziplock bag, add remaining marinade, seal and shake to coat vegetables. Let stand for 1 hour at room temperature.
2. Preheat grill to medium. When heated, oil grates. Place onions, bell peppers and eggplant on grill and cook for 10 minutes. Add zucchini, squash and mushrooms to grill and cook all vegetables about 15 minutes longer, turning halfway through.
3. Cut vegetables into smaller sizes for serving, if desired. Arrange on a platter and serve warm, or cover and chill to serve cold.