Place cabbage in a large colander; pour boiling water over cabbage, and drain well. Set aside.
Sauté onion in butter in a Dutch oven until tender. Add cabbage; cover and cook over low heat 10 minutes.
Dissolve flour in vinegar, stirring with a wire whisk until smooth. Add flour mixture, 1 cup water, sugar, salt, and pepper to cabbage. Cover and cook over low heat 50 minutes or until cabbage is tender.