Cut off beet greens if still attached, leaving a small bit (2 to 3 cm / 1 inch), as well as any roots.
Wash and scrub well.
Boil the unpeeled beet roots just until they are tender. This should be about 20 to 25 minutes in boiling water (depending on size) or try pressure cooker (see Pressure Cooker section below). Don't overcook. You won't want mushy pickled beets.
Meanwhile make up and set aside your spice bag. (See NOTES section below.)
Let beets cool till you can handle them safely, then peel, then cut into slices about 1 cm (¼ inch) thick.
Mix vinegar, water and sugar in a large pot. Add spice bag. Bring to a boil. Add onion (if using) and beets, bring back to a boil then lower to a simmer for 5 minutes.
Remove and discard spice bag.
Use a slotted spoon to divvy the onion (if using) and beet out amongst the jars.
To each half-litre (1 US pint) jar add: ½ teaspoon salt and optionally, ¼ teaspoon Pickle Crisp. (Double those quantities for litre / US quart jars).
Top jars up with pickling liquid.
Leave 2 cm (½ inch) headspace for either size of jar.
Debubble, top up with more liquid as required to retain headspace.
Wipe jar rims.
Put lids on.
Process either size jar in a water bath or steam canner for 30 minutes; increase time as needed for your altitude.