In a large mixing bowl, combine the shredded chicken, diced apple, sliced celery, grape halves, and chopped walnuts.
In a separate small bowl, whisk together Greek yogurt, mayonnaise, Dijon mustard, lemon juice, salt, and pepper until smooth.
Pour the dressing over the chicken mixture and toss gently until everything is evenly coated.
If using any optional ingredients like dried cranberries, fresh herbs, or red onion, fold them in now with a gentle hand.
Taste and adjust seasoning with additional salt, pepper, or lemon juice if needed.
Refrigerate the salad for at least 30 minutes (though 1-2 hours is ideal) to allow flavors to meld before serving.
Give the salad a final gentle toss before serving. Garnish with fresh herbs if desired.
Serve chilled on a bed of greens, in a sandwich, or with crackers.
