Heat butter in large skillet & fry onion over low heat for 5 min or till softened. Add cabbage (should be boiled for 15 minutes previously) & stir over low heat for 2 min. Fold in mashed potatoes till well blended. Press mixture lightly with back of spoon to form large pancake. Cook for 5 min till browned lightly. Turn & brown 2nd side 5 min.