Poach boneless skinless chicken breasts in simmering water or broth for 12–15 minutes until fully cooked, then shred with two forks. Or remove meat from a rotisserie chicken and chop into bite-sized pieces.

Whisk together mayonnaise, Greek yogurt (if using), Dijon mustard, lemon juice, apple cider vinegar, salt, pepper, and garlic powder in a medium bowl. Adjust seasoning to taste.

In a large bowl, mix the shredded chicken, chopped celery, onion, grapes or apple, toasted nuts, and parsley. Pour the dressing over the mixture and gently fold until evenly coated.

Chill the salad for at least 30 minutes to allow flavors to meld. Serve as a sandwich, on a bed of greens, or in lettuce cups. Garnish with additional parsley and cracked black pepper.

Store in an airtight container in the refrigerator for 3–4 days. Keep avocado or apples separate and add just before serving to prevent browning.