Wash cucumbers. Slice very thinly using a food processor, cheese slicer or knife.
Thinly slice capsicum and onion.
Layer cucumbers, capsicum and onion in a baking tray, sprinkling each layer with salt. Cover with a clean tea towel and leave overnight in a cool, dark place.
The next day, add the rest of the ingredients in a saucepan. Stir over medium heat to dissolve sugar. Increase heat and simmer, uncovered, for 5 minutes.
While the pickling mixture is simmering, rinse the cucumbers, capsicum and onion in water and drain off liquid.
Once pickling mixture has simmered for 5 minutes, add the cucumbers, capsicum and onion. Bring the mixture to the boil again and then remove from heat immediately.
Use tongs or a slotted spoon to fill sterilised jar with cucumbers, capsicum and onion. Pour liquid into jar to the top and then seal jar.
Allow to cool and then store in the fridge.