1. Preheat broiler with oven rack 6 inches from heat. Place onion in water to cover; chill until ready to use.
2. Whisk together Caesar dressing and next 5 ingredients.
3. Brush both sides of chicken with 1/3 cup dressing mixture. Place chicken on a lightly greased rack in an aluminum foil-lined jelly-roll pan. Broil 6 to 8 minutes or until browned. Transfer chicken to a serving platter.
4. Place lettuce, cut sides up, on rack in jelly-roll pan; brush with olive oil. Broil 3 to 5 minutes or until browned and wilted.
5. Drizzle desired amount of remaining dressing mixture over chicken and lettuce. Drain onions; pat dry. Serve salad with onions, any remaining dressing, and desired toppings.
Note: We tested with Naturally Fresh Classic Caesar Dressing.