If using raw chicken, cook 2 boneless, skinless chicken breasts in a skillet with a little oil until fully cooked (165°F internal temperature). Let cool, then shred or dice.
In a large bowl, whisk together mayonnaise, sour cream, lemon juice, Dijon mustard, salt, and pepper.
Add shredded chicken, celery, red onion, and parsley to the dressing. Mix well.
If using, fold in walnuts or grapes for extra texture.
Cover and refrigerate for at least 30 minutes to let flavors meld.
Serve on croissants, bread, lettuce wraps, or with crackers.