Bring 3 1/2 pints (2 litres) water to the boil in a large saucepan, adding 2 tbsp salt.

Rinse the bean sprouts; pick out any green seed cases and any wilted sprouts. Add to the boiling water and blanch for only 2-3 minutes. Drain quickly and leave to cool in the colander.

Rinse and dry the cucumber and cut off both ends. Cut in half in the middle of the cucumber. Use a very sharp vegetable knife to remove the skin in strips with about 1/4 in (1/2 cm) of the underlying flesh attached to it. This will bring you close to the central, seed-bearing section which is not used for this recipe. Cut the thick 'peelings' into julienne strips and place them in a large bowl of iced water.

Flake the crab meat coarsely and mix with the bean sprouts and well-drained cucumber strips in a large salad bowl.

Cover and chill for 1 - 1 1/2 hours; transfer to individual plates and make the dressing just before serving: mix the vinegar with 2 pinches of salt and the soy sauce in a bowl.

Beat in the oil, adding a little at a time so that it forms an emulsion, using a small balloon whisk or birch twig whisk. Add freshly ground pepper to taste and serve without delay.