COOK potatoes with 1/3 cup (75 mL) salted water in microwave in a 3 qt (3 L) covered casserole on HIGH for 16 to 18 minutes; stir at half time. Drain & rinse under cold running water to cool.
ADD onions, celery & apples.
BLEND Miracle Whip with mustard; pour over salad & toss to blend. Cover & chill for 2 hours.
GARNISH with pecans.