1.Heat oil in a medium frying pan over moderately high heat. Add chicken; cook for 4-5 minutes each side or until brown and cooked. Transfer to a heatproof plate. Cover with toil; rest for 5 minutes. Slice thinly.
2.Combine apple, celery and juice in a medium bowl. Toss to coat, Add walnuts, parsley, chicken and shredded lettuce.
3.Whisk mayonnaise, mustard and 1 tablespoon water in a small jug. Add dressing to salad. Toss to combine. Arrange remaining lettuce leaves in serving bowls. Spoon salad into bowls. Serve at once.
You can easily turn this into a vegetarian main meal; replace chicken with 4 sliced, hard-boiled eggs and add cubes of cheddar cheese.