Make the dressing: In a large bowl, whisk the mayonnaise, sour cream, lemon juice, mustard, salt, sugar, and pepper.
To the dressing, add the chicken, scallions, celery, dill, and parsley. Toss until evenly combined. Taste and adjust seasoning, if necessary. Cover and refrigerate until ready to serve.
Note: Poached chicken, leftover grilled chicken, or roasted/rotisserie chicken all work well for this recipe. If you'd like to roast bone-in chicken, you'll need 3 pounds bone-in, skin-on chicken breasts. Preheat the oven to 425°F and line a baking sheet with aluminum foil. Arrange the chicken breasts skin-side up on the prepared baking sheet. Coat with 1 tablespoon of olive oil and sprinkle both sides with 1 teaspoon salt and ½ teaspoon pepper. Roast for 30 to 35 minutes, or until done. Let cool before shredding.
Make-Ahead Instructions: This salad can be made and stored in the refrigerator for up to 2 days before serving.
