Prepare the chicken: If using rotisserie chicken, remove skin and bones, then shred or dice into bite-sized pieces. For poached chicken, place 1 lb boneless skinless chicken breasts in a pot, cover with water, add a pinch of salt, bring to a boil, then reduce to a simmer and cook 12-15 minutes until cooked through. Let cool, then shred.
Make the dressing: In a large bowl, whisk together mayonnaise, sour cream, lemon juice, Dijon mustard, garlic powder, onion powder, salt, and pepper until smooth.
Add herbs and aromatics: Stir in parsley, dill, chives, celery, and red onion (if using).
Combine: Add the chicken to the bowl and fold gently until evenly coated. Taste and adjust salt, pepper, or lemon juice as needed.
Chill: Cover and refrigerate for at least 30 minutes to let flavors meld. Serve cold or at room temperature.