Heat oven to 375ºF. Cook potatoes in boiling water in saucepan 15 min. or until tender. Meanwhile, cook onions in medium skillet sprayed with cooking spray 3 min., stirring frequently. Add salmon, cream cheese product, Miracle Whip
, milk and nutmeg; cook 2 min. or until cream cheese is melted and mixture is blended, stirring frequently. Stir in spinach; cover. Cook 3 min. or just until spinach starts to wilt; stir.