Peel and quarter potatoes. In covered large saucepan, cook in small amount of boiling lightly salted water for 20 to 25 minutes or until tender. Drain. Mash (should have about 6 cups). Add sour cream, butter, chives, and pepper; mash again until butter is melted and mixture is smooth. Stir in spinach. Transfer to a 2-quart casserole. Bake, uncovered, in a 400 degree F oven for 15 to 20 minutes or until heated through. Sprinkle with cheese. Makes 12 servings.