In a large bowl, whisk together the mayonnaise, sour cream, lemon juice, mustard, salt, sugar, and pepper until smooth.

Add the shredded chicken, sliced scallions, diced celery, chopped dill, and parsley to the dressing.

Toss the mixture thoroughly until all ingredients are evenly coated with the dressing.

Taste the salad and adjust the seasoning with additional salt or pepper if necessary.

Cover the bowl and refrigerate until ready to serve. The salad can be made up to 2 days in advance.