Have some hard-boiled eggs; cut them in two lengthways. Cook in butter a few fresh mushrooms for about ten minutes; season them with salt, pepper, and a little lemon juice. Chop together finely the mushrooms and the yolks of eggs, mix with Béchamel sauce so that the stuffing is the consistency of a stiff sauce. Fill the eggs, which you put in a buttered fireproof dish, with, if you like, a little Béchamel sauce around the eggs. Brown and serve at once. The proportions should be about a quarter of a pound of mushrooms for six eggs.