In a large nonreactive (glass, ceramic or plastic) bowl, toss the cucumber with onion, salt and hot water and set aside for 3 hours. Drain, reserving the liquid.
In a medium saucepan, pop the vinegar, sugar, mustard seeds, dill, turmeric and chilli (if using). Bring to a boil over high heat, add the cucumber and its reserved soaking liquid, then reduce the heat to low and simmer for 2 minutes.
Transfer the pickles to sterilised jars. Store for a month in the fridge, ideally, before using to let the flavour develop.