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).
Recipes with the "Healthy Pick" stamp have been evaluated by a registered dietitian member of the Ordre professionnel des diététistes du Québec.