If using rotisserie chicken, shred meat into bite-sized pieces (about 3 cups total). Let cool completely.
Toast chopped nuts in a dry skillet over medium heat for 3-4 minutes until fragrant and golden. Set aside to cool.
Dice celery into ¼-inch pieces. Halve grapes. Finely chop red onion if using.
In a large bowl, whisk together mayo, Greek yogurt, lemon juice, Dijon mustard, garlic powder, salt, and pepper.
Add chicken, celery, grapes, toasted nuts, and onion to the bowl. Fold gently until evenly coated.
Cover and refrigerate for at least 30 minutes before serving. Store in an airtight container for up to 4 days.
Serving Suggestions: Serve on croissants, with crackers, in lettuce wraps, or on a bed of greens.
