Prep the chicken: If using rotisserie, remove skin and shred the meat finely (mixing light and dark meat gives better flavor). For poached: simmer salted water, add chicken breasts, cook 12–15 minutes to 165°F/74°C, cool, then shred.
Make the dressing: In a large bowl whisk Greek yogurt, mayo, Dijon, lemon juice, salt, and pepper until smooth. Taste and adjust salt/lemon now.
Fold & balance: Add chicken, celery, red onion, and herbs. Fold gently until everything is evenly coated. Stir in any add-ins you choose (grapes/almonds, etc.).
Chill (optional, recommended): Cover and refrigerate 20–30 minutes so flavors marry and the dressing clings.
Serve: Spoon onto greens, pile into a sandwich or croissant, or scoop with crackers. Finish with extra herbs and a squeeze of lemon if you like it brighter.