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Pour the vinegar into a pan, add the sugar, sea salt, the spices and bay leaf, and bring to a simmer. After 1 min, check that the sugar and salt have dissolved. Remove from the heat.
Boil the kettle. Put the onion slices in a sieve or colander. Pour over the hot water from the kettle and drain well. When cool enough to handle, pack the onion rings into a 500g sterilised jar (see tip below). Pour over the warm vinegar and seal. Cool, then chill and leave to pickle for 2 hrs. Will keep for 6 months unopened, or 2 weeks in the fridge once opened.