1. Cook cider vinegar,sugar,and water in medium saucepan over medium heat,stirring often,until Sugar is dissolved. Add mustard seeds. Cook 20 minutes or until seeds are soft but slightly firm. Strain seeds reserving liquid;set seeds aside. Simmer reserved liquid until syrupy and reduced by half. Pour syrup over seeds; set aside and cool 2. Place potatoes in large bowl, set aside. Whisk sour cream,next add next 6 ingredients and 1 1/2 tsp salt in a medium bowl . Pour mixture over potatoes . Toss gently. 3. Stir celery and next 3 ingredients ,2 tbsp mustard seed and remaining salt into potato mixture. Cover and chill3 to 4 hours. Garnish with celery leaves..