Make the dressing: In a large bowl, whisk the Greek yogurt, mayonnaise (if using), lemon juice, lemon zest, Dijon, garlic powder, salt, and pepper until smooth.
Add herbs and veggies: Stir in the dill, celery, and red onion. If using cucumber or capers, add them now.
Fold in the chicken: Add the chopped or shredded chicken and gently mix until everything is evenly coated.
Avoid overmixing to keep the texture light.
Taste and adjust: Add extra salt, pepper, or lemon to taste. If it’s too thick, loosen with a splash of water or a bit more lemon juice.
Chill (optional but recommended): Cover and refrigerate for 20–30 minutes to let the flavors meld.
Serve your way: Spoon into lettuce cups, pile onto toasted bread, tuck into a pita, or serve with crackers and sliced veggies.