Whisk together the buttermilk, mayonnaise, chives, vinegar, sugar, lemon juice and 1/2 teaspoon each salt and pepper in a small bowl. Combine the walnuts, celery and apples in a separate large bowl. Pour three-quarters of the dressing over the celery-apple mixture and toss to coat. Let stand for 5 minutes at room temperature, then toss again. Adjust the seasoning as desired. Add the remaining dressing and toss again. Serve at room temperature.