Prepare the chicken by boiling, roasting, or grilling until fully cooked and tender. Let it cool, then dice into bite-sized pieces.
Chop the celery and halve the red grapes. If using green onions, slice them thinly.
In a small bowl, mix together mayonnaise, Dijon mustard, lemon juice, garlic powder, salt, and black pepper until smooth.
In a large mixing bowl, combine diced chicken, chopped celery, and halved grapes. Pour the dressing over and stir gently to coat. Fold in walnuts and green onions if using.
Cover the bowl with plastic wrap and refrigerate for at least 60 minutes to allow flavors to meld.