BOIL the lobster in a court-bouillon according to the directions. Cook the mushrooms according to the basic recipe; they should, however, be sliced rather than left whole.

While the lobster is cooking, in a heavy pan, over low heat, cook the chopped onions slowly in the butter until they are very soft but not at all brown. In another pan, simmer together the cream and CONCENTRATED FISH STOCK until the mixture thickens.

Then strain the butter from the onions into this sauce, and discard the onions. Bring the sauce back to the boiling point, stirring well. Now, off the fire, with a sauce whisk beat in the egg yolks, one at a tirne, and the mustard. Beat well until the sauce is smooth and thick. Season it very lightly with salt and pepper.

As soon as the lobster is done, remove it from the kettle. Separate the claws from the body and crack them open; cut off and split the joints with scissors. Extract all the meat. Place the lobster on its back on a wooden board, and cut it lengthwise completely in two with a strong sharp knife, running it from the top of the body down through the rail. Remove the rail meat and empty the cavity of the shells.