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Put the cucumbers, onion slices and salt in a sieve set over a bowl, and cover with ice and something to weigh it down. Let stand for 1-2 hrs in the fridge, then drain well. (This helps to push the water out of the cucumber, which means it will stay crunchy later when brined.)
Meanwhile, put the vinegar, 200ml water, the sugar, turmeric, celery seeds, chilli flakes and mustard seeds in a small saucepan over a medium heat. Bring to the boil and simmer for 5 mins. Remove from the heat and leave to cool to room temperature. Put the cucumber and onion mixture in a 500ml- capacity jar or sealable plastic container, pour over the brining liquid, cover and seal. Chill for at least 3 hrs. Will keep in the fridge for 2 weeks.