Peel the lotus root and cut it into slices about 2mm thick (using a mandolin grater is best for this). Place in a large bowl of cold water and rinse thoroughly, then leave a gentle flow of cold water running through it while you prepare the rest.
Place everything else in a pan with 450 ml water and bring to the boil. Fill a heatproof preserving jar with boiling water and leave to sit for a minute on a folded tea towel (it helps, trust me). Tip the hot water from the jar, drain the lotus root slices and place in the jar, then pour as much of the boiling liquid over them as you can, adding as much of the ginger as possible, and seal while hot. If you are recycling a jar, it is important to use a new seal or a very tight-fitting lid. Leave to cool on the tea towel, then place in the fridge. Leave for at least 3 days before using.