If using raw chicken, place 1 pound boneless skinless chicken breasts in a pot, cover with water, add a pinch of salt, and bring to a boil. Reduce heat and simmer for 12-15 minutes until cooked through. Let cool, then shred with two forks. Alternatively, shred a store-bought rotisserie chicken.
In a large bowl, whisk together mayonnaise, Greek yogurt, Dijon mustard, lemon juice, salt, and pepper until smooth.
Add the shredded chicken, celery, red onion, parsley, dill, and chives. Stir gently until everything is evenly coated.
Taste and adjust seasoning—add more lemon juice or salt if needed. If using nuts, fold them in now.
Cover and refrigerate for at least 30 minutes to let the flavors meld. Serve chilled or at room temperature.