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 (1/2 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.