Wash and clean the cucumbers first. Then slice them into 3-4 mm thickness (pound coin thickness) with a sharp knife or a vegetable slicer mandoline (to get the nice ridges).
Peel and slice the onion thinly.
In a plastic (or glass) bowl mix the cucumbers, onion, dill, mustard seeds and pepper.
In a measuring jug mix together the sugar, salt and the vinegar and stir until the sugar has dissolved. Pour over the cucumber mix and leave for a minimum of 4 hrs (but better over night 10-12 hrs.)
When you are ready, pour everything into a large jar (or two smaller ones), seal with lid and store in the fridge for up to 2-3 weeks.
The pickles are ready to be eaten straightaway, but their flavour will improve with time.