Make the dressing: In a large bowl, whisk the Greek yogurt, Dijon, lemon juice, lemon zest, olive oil, garlic, salt, and pepper until smooth.
Add the crunch: Stir in celery, onion, and herbs. If using grapes or apple, fold them in now.
Add chicken: Mix in the shredded or chopped chicken until evenly coated.
Taste and adjust salt, pepper, and lemon as needed.
Chill: Cover and refrigerate for at least 20–30 minutes. This helps the flavors meld and the yogurt firm up.
Serve: Spoon onto toast, stuff into pita or a wrap, or pile over mixed greens. Add nuts or capers on top for extra texture and flavor.