In a soup kettle or Dutch oven, cook the potatoes and carrots in boiling salted water until tender; drain. Place in a bowl; mash and set aside. , In a skillet, saute onions in butter until tender. Add to potato mixture. Add sour cream, salt and pepper; mix until blended. , Transfer to a greased 13x9-in. baking dish. Sprinkle with cheese. Bake, uncovered, at 350degrees for 30-40 minutes or until heated through. <br><b>Freeze option:</b> Cool unbaked casserole; cover and freeze. To use, partially thaw in refrigerator overnight. Remove from refrigerator 30 minutes before baking. Preheat oven to 350degrees. Bake casserole as directed, increasing time as necessary to heat through and for a thermometer inserted in center to read 165degrees.