Remove meat from rotisserie chicken, discarding skin and bones. Shred or chop into bite-sized pieces; you should have about 3 cups.
In a large bowl, whisk together mayonnaise, Greek yogurt, Dijon mustard, lemon juice, salt, pepper, and garlic powder until smooth.
Add shredded chicken, celery, red onion, dried cranberries, and nuts (if using). Stir gently until evenly coated.
Taste and adjust seasoning with more salt, pepper, or lemon juice as needed.
Cover and refrigerate for at least 30 minutes before serving to allow flavors to meld.