Place the diced, chilled chicken in a large mixing bowl with the chopped celery, halved grapes, and chopped green onions.
In a medium bowl, whisk together the mayonnaise, sour cream, lemon juice, mustard, chopped parsley, dried dill, onion powder, garlic powder, sugar, salt, and black pepper until smooth.
Pour the dressing over the chicken mixture and fold gently until everything is evenly coated; stir in toasted almonds or pecans if using.
Cover the bowl and chill in the refrigerator for at least 4 hours to allow flavors to meld before serving.
Serve on sandwiches, croissants, or in wraps with lettuce, or on a bed of lettuce with tomato and avocado for a low-carb option.