Have ready hot, sterilized jars and their lids.
Put the beets into a large saucepan (if using different-colored beets, separate them into 2 saucepans) and add water to cover by
When the cooked beets are cool enough to handle, peel them and then cut into slices ¼ inch (6 mm) thick. Divide the beet slices and the onion slices among the jars.
In a saucepan over medium heat, combine the reserved cooking liquid, the vinegar, sugar, cardamom, cloves, and salt. Bring to a boil, stirring, just until the sugar is dissolved.
Ladle the hot vinegar mixture into the jars, evenly distributing the spices and leaving
Process the jars for 7 minutes in a boiling-water bath (for detailed instructions, including cooling and testing seals, Canning Step by Step). Let the jars stand undisturbed for 24 hours and then set them aside for 1 week for the flavors to develop. The sealed jars can be stored in a cool, dark place for up to 3 months. If a seal has failed, store the jar in the refrigerator for up to 1 week.