Mix the salad: In a large bowl, combine the chicken, grapes, green onions, celery, bread-and-butter pickle slices, dill, parsley, mustard seeds, walnuts, mayonnaise, Greek yogurt (or sour cream), and lemon juice. Stir gently until everything is evenly coated.
1 to 2 cups red seedless grapes, 3 green onions, 2 stalks celery, 3 bread and butter pickle slices, ¼ cup fresh dill, ⅓ cup fresh parsley, ½ tsp whole mustard seeds, ½ cup chopped walnuts, ½ cup mayonnaise, ½ cup plain Greek yogurt or sour cream, 2 Tbsp fresh lemon juice