Mussel Soup a la Vigo

For Ten Persons.—Take two ounces or one medium sized onion, cut it in one-eighth of an inch squares, and fry brownless in some butter, add to this sixty medium mussels previously cooked in very little water and white wine (no salt), parsley, thyme and bay leaf; when the shells open they are done, remove them from the shells; from each one remove the black and nervous part, being careful not to break them while doing so. Strain the broth, let it settle so as to be able to pour off the clear top, put a little of the broth with the mussels and keep them warm. Put the rest of the broth in a saucepan with two quarts of thickened lean fish stock (No. 195). Boil and despumate for ten minutes, then thicken with six egg-yolks, diluted in half a pint of cream and four ounces of fine butter. Put into the soup tureen four ounces of mushrooms cut in small Julienne, also the mussels; pour the boiling soup over and serve.