In a saucepan, place raw beets and enough water to cover. Cook until tender, 15-20 minutes. Drain, reserving 1/4 cup liquid. (If using canned beets, drain and reserve 1/4 cup juice.) , In another saucepan, combine sugar, flour, vinegar and reserved beet juice. Cook over low heat until thickened. Stir in beets, salt and butter. Simmer for 10 minutes.