Grilled vegetables work best. If you like, you can make this dish ahead and serve it cold.
To prepare dressing, combine first 6 ingredients in a bowl; stir well with a whisk.
Prepare grill or broiler.
To prepare salad, combine tomatoes and dressing, and set aside. Sprinkle eggplant with 1/8 teaspoon salt. Place eggplant on a grill rack or broiler pan coated with cooking spray; cook 5 minutes on each side or until eggplant is done. Remove eggplant from grill rack or pan; set aside. Sprinkle onion with 1/8 teaspoon salt. Place onion on grill rack or broiler pan coated with cooking spray; cook for 5 minutes on each side or until onion is tender.
Arrange eggplant and onion slices on 6 plates. Top with tomato mixture; sprinkle with mint and basil.