Wash Brussels sprouts. Remove stems, rough-looking outer leaves. Any that are larger than 5 cm (2 inches), cut in half (through the stem so they will stay together.) Blanch for 4 minutes, counting the time from the time from when the water comes back to the boil. Plunge into cold water; let cool them drain well.
In a large pot, combine the onion, pepper, vinegar, mustard seed, celery seed, turmeric, red pepper flakes, and sugar. Bring to a boil, then lower heat and simmer for 5 minutes.
Divide the Brussels amongst the jars you are using: either quarter-litre (½ US pint) or half-litre (1 US pint) jars.
Leave about 3 cm (1 inch) free at the top of each jar.
At this step, you will know if you need extra jars.
[Optional] Add a cardamom pod and 2 or 3 peppercorns to each jar.
[Optional] Add an ⅛th teaspoon Pickle Crisp to each jar.
Using a slotted spoon, divide the onion and pepper amongst the jars.
Leave 2 cm (½ inch) headspace
Fill the jars with the brine. Leave 2 cm (½ inch) headspace.
Debubble, adjust headspace.
Wipe rims.
Put lids on.
Process in a water bath or steam canner.
Process either size jar 10 minutes; increase time as needed for your altitude.