Place the potatoes in a saucepan and cover with water; cover and bring to a boil over medium-high heat. Cook for 15-20 minutes or until very tender. Drain well and place in a bowl; mash. Add the sour cream, butter, sugar, salt, pepper and dill; mix well. Stir in spinach.
Transfer to a greased 2-qt. baking dish. Sprinkle with cheese. Bake, uncovered, at 350° for 30-35 minutes or cheese is melted.