Cook cabbage, covered, in a small amount of boiling salted water for 15 minutes; drain well, and set aside.
Sauté onion in butter until tender. Combine cabbage and onion; add salt and pepper. Cook over low heat 2 minutes. Fold in mashed potatoes until well blended.
Spoon mixture onto a hot, lightly greased 9-inch griddle; pat mixture evenly on griddle, leaving a 1/2-inch margin around edge of griddle.
Cook 20 minutes or until both sides are browned, turning once.
Oxmoor House Homestyle Recipes