Peel the cucumbers thinly with a potato-peeler. Cut them in half lengthways, and hollow out the seeds with a teaspoon. Then, slice the cucumber halves as finely as possible. Put the slices in a bowl and cover them with 3tablespoons of coarse salt (fine salt would do instead). Mix in well and leave for at least 2½ hours. The quantity of salt may seem enormous to you but it will remove all the surplus water from the cucumbers and will be strained off with the water when they are drained.
Meanwhile, prepare the cream sauce. Put the chilled cream, mustard and lemon juice in a bowl and whip till thick and foamy. Add the chervil and 3 or 4 turns of the pepper-mill. When you are whipping the cream, do not overdo it or you will end up with butter. When the cream clings to the wires of the whisk or to the beater, it is time to stop.
Put the cucumbers to drain in a colander, pressing them down to get rid of all the liquid. Mix them into the whipped cream, taste for seasoning and serve very cold.