Slice the chicken into thin strips and brush all over with the olive oil. Heat a griddle pan and cook the chicken for 3-4 minutes, then turn and cook for another 1-2 minutes on the other side, or until cooked through. You may have to do this in batches. Set the cooked chicken aside and allow to cool slightly.
Toss together the celery, cucumber, grapes and cooked chicken. Spoon on to the lettuce leaves and then top with the chicken. Drizzle over the dressing to serve.