1.Place potatoes in a 3.qt.saucepan add enough water to cover.Heat to boiling;reduce heat.cover and cook 20 to 25 minutes or until potatoes are tender.Drain and return to saucepan.
2.Heat potatoes over low heat about 1 minute,shacking pan often to keep potatoes from sticking and burning.Mash potatoes until no lumps remain.
3.In 1.-qt saucepan,heat milk,whipping cream,margarine salt and pepper over medium-low heat,stirring occasionally,until margarine is melted;reserve and refrigerate 1/4 cup mixture.Add remaining milk in small amounts to potatoes mashing after each addition,until potatoes are light and fluffy. Spray 2.Qt.casserole,cover and refrigerate up to 24 hours.
Heat oven to 350 degrees.Uncover casserole;pour reserved milk mixture over potatoes.Bake at 40-45 milk mixture over potatoes.bake 40-45 minutes or until hot.Stir potatoes before serving.