Remove tops from 3 1/2 lbs. beets, leaving 1 1/2 inch of stem. Cook until tender. Drain, saving 1 cup of liquid. Remove skins, roots, stems. Slice. Combine 1 cup liquid, 4 cups vinegar, 1 cup sugar. Add 1/3 cup mixed pickling spices tied loosely in cheesecloth bag. Bring to a boil. Add beets and 3 medium onions, sliced. Boil 5 minutes. Remove spice bag. Pack in jars and seal. 6 pints.