Broil peppers 4 in. from the heat until skins blister, about 10 minutes. With tongs, rotate peppers a quarter turn. Broil and rotate until all sides are blistered and blackened. Immediately place peppers in a bowl; cover and let stand for 15-20 minutes.
Peel off and discard charred skin. Remove stems and seeds. Cut peppers into thin slices. In a large bowl, combine the peppers, mushrooms, onion and olives.
In a small bowl, combine the salad dressing, garlic, basil, salt, oregano and pepper. Pour over vegetables; toss to coat. Cover and refrigerate overnight. Serve on lettuce leaves.