Prep the chicken: If using rotisserie or leftover chicken, remove skin and bones. Chop into small, even pieces.
For poached chicken, simmer salted water, add chicken breasts or thighs, cook until just done, cool, then chop.
Make the dressing: In a large bowl, whisk together mayonnaise, Greek yogurt, Dijon, lemon juice, sugar, salt, pepper, garlic powder, and celery seed until smooth.
Add the crunch: Stir in celery, red onion, relish or pickles, and parsley. Mix until evenly coated.
Fold in the chicken: Add the chopped chicken and gently fold until everything is well combined. If using grapes, nuts, or egg, fold them in now.
Taste and adjust: Add more salt, pepper, or lemon as needed.
If it feels a bit thick, a teaspoon of water or extra lemon juice loosens it up.
Chill to meld: Cover and refrigerate for at least 30 minutes. This helps the flavors settle into that deli-style balance.
Serve your way: Spoon onto croissants, toast, or a soft deli roll. Or scoop over mixed greens, tuck into lettuce cups, or serve with crackers and sliced cucumbers.