Make the dressing: In a large bowl, whisk the Greek yogurt, mayonnaise (if using), Dijon, lemon juice, lemon zest, garlic, dill, salt, and pepper until smooth.
Add the mix-ins: Stir in the celery, red onion, grapes, and nuts until evenly coated.
Fold in the chicken: Add the cooked chicken and gently mix until everything is well distributed.
Taste and adjust salt, pepper, or lemon.
Chill briefly: Cover and refrigerate for 15–30 minutes to let the flavors meld. This step makes a big difference.
Serve your way: Spoon into lettuce cups, pile onto toasted sourdough, stuff into a croissant, or serve over mixed greens.