Top and tail the beans in the usual way. Wash them and drain in a colander. Bring 3litres (5¼pints) of water with 2tablespoons of coarse salt to the boil and throw in the beans. Do not cover. When they have cooked for 4–5 minutes take one out from time to time and test it between your teeth to see if it is done. (It is impossible to give an exact cooking time because so much depends on the calcium content of the water and above all on the quality of the beans.) Meanwhile, get ready a bowl of cold water and ice cubes. When the beans are still just slightly crisp, drain them quickly and plunge into the cold water. This operation stops the cooking and keeps them beautifully green. Drain carefully, if necessary drying them on a cloth, and keep in a cool place.
Cut the hazelnuts into thin slices with a small sharp knife and toast lightly in a small cast-iron frying pan over a brisk heat, stirring all the time. They should not brown, but remain pale. This operation can also be done in the oven (or under an ordinary grill). Put the nuts on a plate to cool.
Take the chilled bowl out of the refrigerator, and put the chilled cream, mustard, lemon juice, salt and pepper into it. Whisk lightly to mix the ingredients thoroughly, then add the chopped chervil (if used) and the beans. Turn the beans over in the cream with a spoon. Taste the dressing and add salt if necessary. This operation should not be carried out more than half an hour before the meal.
In each of two deep plates put two lettuce leaves like open hands. Pile the beans, in their dressing, on the lettuce in two mounds, and finish with a sprinkling of grilled hazelnuts (2). Serve.