Prepare the chicken: If using rotisserie chicken, remove skin and bones, then shred into bite-sized pieces. If poaching, place 2 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 for 12-15 minutes until cooked through. Let cool, then shred.
Make the dressing: In a large bowl, whisk together mayonnaise, sour cream, lemon juice, and Dijon mustard until smooth.
Add herbs and vegetables: Stir in parsley, dill, chives, celery, and red onion.
Combine: Add shredded chicken to the bowl and mix gently until evenly coated. Season with salt and pepper.
Adjust: Taste and add more lemon juice or herbs if desired. Fold in nuts if using.
Chill: Cover and refrigerate for at least 30 minutes to let flavors meld.