Prepare Ingredients: If not already done, cook and shred or dice the chicken. Halve the grapes, chop the walnuts, celery, and red onion.
Mix Dressing: In a large bowl, combine the mayonnaise, Greek yogurt (if using), Dijon mustard, and lemon juice. Stir well until smooth and well mixed.
Combine Salad: Add the chicken, grapes, walnuts, celery, and red onion to the bowl with the dressing. Mix gently until everything is evenly coated with the dressing.
Season: Taste and adjust seasoning with salt and pepper as needed.
Chill: Cover the bowl and refrigerate for at least an hour to allow the flavors to meld together.
Garnish (Optional): Before serving, sprinkle with fresh parsley if desired.
Serve: Serve the salad on a bed of greens, as a sandwich filling, or with crackers.