Wash the cucumbers thoroughly and slice them thinly and evenly, using a sharp knife or mandoline. Peel the onions and slice them very finely.
Place the cucumber and onion into a colander and toss with the salt, making sure the vegetables are evenly coated. Fit the colander into a large bowl to catch the excess liquid.
Cover the surface of the vegetables with a piece of baking parchment, then place a heavy weight on to help draw out the excess liquid.
Transfer to the fridge and chill for at least 2 hours or overnight if you prefer.
Drain away the liquid released by the cucumber and onion and spread them out on a wire rack in an even layer. Pat them dry using a paper towel, a clean kitchen towel or some clean muslin. Allow them to rest and dry a little further whilst you prepare the pickling vinegar.
Place the rest of the ingredients (except for the fresh dill) in a non-reactive saucepan (stainless steel is ideal as it won’t react with the vinegar). Set the pan over a medium heat and bring the mixture to a gentle boil, stirring occasionally to help the sugar to dissolve. Reduce the heat and simmer for 5 minutes until the sugar has completely dissolved and the spices are releasing their flavour.
Carefully tip the drained cucumber and onion into the hot pickling liquid and bring it up to the boil. As soon as it reaches boiling point then remove from the heat.
Add the fresh dill into the pan and stir to distribute it throughout the ingredients.
Use clean tongs and remove the vegetables the pan to pack into hot sterilised jars, distributing evenly with the dill. Leave approximately 1-1.5cm (½ inch) headspace towards the top of the jar/s.
Pour over the pickling vinegar and spices to cover the vegetables in the jars. Tap on the jars to release any trapped air bubbles. Wipe the rims of the jar/s with a clean cloth then seal with sterilised lids.
All the jar/s to cool completely then transfer to the refrigerator to keep chilled. Allow the bread and butter pickles to rest for at least 24 hours to allow the flavours to settle then enjoy.
Store the pickles in the fridge and use them within 2 weeks.