Make the dressing: In a large bowl, whisk Greek yogurt, mayonnaise (if using), Dijon, lemon juice, honey, garlic powder, onion powder, salt, and pepper until smooth. Taste and adjust seasoning.
Prep the mix-ins: Dice the celery, mince the onion, halve the grapes (or dice the apple), and chop the nuts and herbs.
Add the chicken: Fold the chopped or shredded chicken into the bowl with the dressing until coated.
Stir in crunch and color: Add celery, onion, fruit, nuts, and herbs.
Gently mix to combine without mashing the chicken.
Adjust to taste: Add a pinch more salt, a squeeze of lemon, or an extra spoon of yogurt if it needs more creaminess. If it tastes flat, a small splash of lemon and a tiny pinch of salt usually does the trick.
Chill (optional but recommended): Cover and refrigerate for 20–30 minutes. This helps the flavors meld and the texture firm up.
Serve: Spoon onto toasted sourdough, stuff into a pita, wrap in lettuce leaves, or serve over mixed greens with cucumber and tomatoes.