Cut tomatoes into 1/4-inch-thick slices. Arrange on an aluminum foil-lined baking sheet coated with cooking spray.
Broil 5 minutes on each side; set tomato aside.
Cut bell peppers in half lengthwise; remove and discard seeds.
Arrange peppers (cut sides down) and onion on a foil-lined baking sheet coated with cooking spray.
Broil, 5 inches from heat, 8 to 10 minutes on each side or until peppers look blistered. Remove peppers, and broil onion 3 more minutes, if necessary.
Place peppers in a heavy-duty zip-top plastic bag; seal and let stand 10 minutes to loosen skins. Peel peppers; cut into thin strips.
Whisk together oil and next 5 ingredients in a large bowl; add vegetables, tossing to coat. Sprinkle with basil.
