Preparation - Mash the potatoes. Add the sour cream, sugar, butter or margarine, milk, salt and pepper. With an electric mixer at medium speed, beat until light and fluffy. Add the chives and the well-drained, cooked spinach and mix well. Pour the mixture into a buttered casserole dish. Bake, uncovered, in a preheated oven at 400º for 15 minutes. Sprinkle the shredded cheese on top and bake for an additional 10 minutes.