Place chicken in a small saucepan with enough water to cover; bring to the boil. Reduce heat to low-medium; cook for 7 minutes or until just cooked through. Cool in poaching liquid. When cool enough to handle, shred thickly.
Meanwhile, make dijon mayo dressing.
Using a mandoline or V-slicer, thinly slice apple and onion.
Place apple and juice in a medium bowl; toss to coat. Add shredded chicken, celery, witlof, walnuts, onion, parsley, grapes and the dressing; toss gently to combine. Season to taste.
Serve salad on a large platter with remaining dressing and extra grapes.
Dijon mayo dressing
Place ingredients in a screw-top jar; season to taste. Shake well.