Place the cucumber slices, onion and salt in a large, non-metallic bowl and mix well. Cover with clingfilm and chill for 2 hours. Rinse and drain well, then transfer to a clean bowl.
Sterilize your jars and lids. Pack the cucumbers and onion into the jars so they fit snugly.
Put the seeds, vinegar, water and sugar in a small saucepan. Bring to