1. Trim beets, leaving 1 inch of stem, and scrub. Bring beets to a boil in water to cover in a medium saucepan; reduce heat, and simmer 25 to 30 minutes or until tender. Drain, rinse, and cool slightly. Trim off roots and stems; peel beets. Cut beets in half vertically; cut halves into 1/4-inch-thick slices to measure 6 cups.
2. Sterilize jars, and prepare lids.
3. While jars are boiling, stir together vinegar, next 4 ingredients, and 1 1/4 cups water in a stainless steel or enameled 8-qt. Dutch oven. Bring mixture to a boil. Add beets and onions; reduce heat, and simmer 5 minutes. Remove and discard spices.
4. Using a slotted spoon, divide beets and onions evenly among hot jars, leaving 1/2-inch headspace. Cover beet mixture with hot pickling liquid, leaving 1/2-inch headspace. Seal and process jars, processing 30 minutes.
5. Remove jars from water, and let stand, undisturbed, at room temperature 24 hours. To check seals, remove the bands, and press down on the center of each lid. If the lid doesn't move, the jar is sealed. If the lid depresses and pops up again, the jar is not sealed. Store properly sealed jars in a cool, dark place up to 1 year. Refrigerate after opening.
Total time: 1 hour, 55 min., plus 3 weeks standing time