If using raw chicken breasts, season with salt and pepper, then poach in simmering water or broth for 12-15 minutes until cooked through. Let cool slightly, then shred or dice. For rotisserie chicken, simply remove meat from bones and chop into bite-sized pieces.
In a large bowl, whisk together mayonnaise, Greek yogurt (or sour cream), lemon juice, salt, pepper, and any optional spices until smooth.
Add the chicken, celery, red onion, parsley, and dill to the bowl. Fold gently with a spatula until everything is evenly coated. Taste and adjust salt or lemon as needed.
For best flavor, cover and refrigerate for at least 30 minutes before serving. This lets the herbs meld and the chicken absorb the dressing.
Serve chilled or at room temperature. Enjoy on bread, crackers, lettuce cups, or straight from the bowl.