Using a mandolin or sharp knife, thinly slice the cucumber and onion into a large bowl, and stir in the chopped fresh dill.
Then pour the sugar, cider vinegar and sea salt over the cucumber and onion. Stir well and cover with a towel (or cling film) and leave for about two hours.
Give the cucumber pickle a good stir and leave for one hour. Either store in an airtight container or screw top jar in the fridge or bottle in a sterilised jar. Keeps for a month.