In a large bowl, whisk together the mayonnaise, soured cream, lemon juice and sugar until combined.
Core the apple and slice into thin wedges. Add to the dressing along with the grapes and celery; toss to coat.
Tear the chicken into bite-size pieces then add to the bowl, mixing together until combined. Season.
Tear the romaine lettuce into bite-size pieces and arrange on plates. Top with the chicken mixture, a sprinkling of walnut pieces and chopped fresh parsley.
Cook's tip: For an easy midweek lunch for two, simply double the chicken salad mixture and stuff it into wholemeal pitta breads.