1. Heat gas grill to medium-high or prepare charcoal grill with medium-hot coals.
2. In small bowl, whisk together balsamic vinegar, garlic, basil, salt and pepper. Slowly whisk in olive oil; set aside. Place romaine in a large serving bowl.
3. Place peppers on grill and cover; grill for about 6 to 7 minutes per side or until skin is starting to turn black. Remove from grill and cut into 1/2-inch pieces; add to romaine in bowl.
4. Grill corn 10 minutes, turning, or until tender and beginning to brown. Remove corn from grill. Cut kernels from cob; add corn to romaine. Starting at root end, cut red onion into 1/2-inch-thick slices and secure with toothpicks to keep rings from separating. Grill slices 5 minutes per side, turning once. Remove and cut into 1/2-inch pieces; add to romaine mixture.
5. Toss salad with prepared dressing and serve immediately.