Melt butter in a large skillet.  Slice the turnips and place them in the skillet.  Add 1/3 cup water, sprinkle turnips with sugar substitute and salt then cover with a lid and cook over medium heat 15 minutes, until tender.Uncover skillet and continue cooking until all the water is evaporated. Stir in cream; cook until cream thickens slightly and turnips are evenly coated, about 3 minutes. Add pepper just before serving.