- In a large pot of boiling water, cook the beets, covered, until they are tender, about 1 hour. Set aside 1 cup (250 ml) of filtered cooking water. Plunge the beets into cold water. Drain, peel and cut the beets into quarters or slices. Set aside.
- In another saucepan, bring the reserved cooking water, vinegar, sugar, mustard seeds and salt to a boil.
- Put the beets into sterilized jars and cover with the boiling marinade. Seal. Sterilize if desired (35 minutes in boiling water).