Pour the cider vinegar into a saucepan, then add the golden caster sugar, sea salt flakes, black peppercorns, coriander seeds, star anise, and bay leaf. Bring this mixture to a gentle simmer. After one minute, check to ensure the sugar and salt have fully dissolved. Take the pan off the heat.

Boil a kettle and pour the hot water over the onion rings placed in a sieve or colander, ensuring they are well-drained. Once the onions are cool enough to handle, pack them tightly into a sterilized 500g jar. Pour the warm vinegar mixture over the onions, making sure they are fully submerged, and then seal the jar. Allow it to cool to room temperature before refrigerating. Let the onions pickle for at least two hours before using. Unopened, they can be stored for up to six months, but once opened, they should be consumed within two weeks if kept in the fridge.