Put the cucumber in a colander over a bowl, sprinkle with the sea salt and mix well. Leave to stand for 1 hour to draw out any excess moisture and then rinse well under cold running water and drain.
Heat a wok until smoking. Add the peanut oil and, when hot, add the peppercorns and chillies and cook until blackened.
Add the sugar, vinegar, soy sauce, ginger, mushrooms and finally, the cucumber. Stir for a minute or two. Pack into a 2 litre (8 cup) sterilised glass jar, allow to cool and then cover and leave for a day or two to mature before using.