This section is from the book "Studies of American Fungi: Mushrooms, Edible, Poisonous, Etc.", by George Francis Atkinson. Also available from Amazon: Studies of American Fungi: Mushrooms, Edible, Poisonous, Etc..
Wash and dry the mushrooms; cut them into strips crosswise of the gills, trimming off all the woody portion near the stem side. Throw the mushrooms into a saucepan, allowing a tablespoonful of butter to each pint; sprinkle over a half teaspoonful of salt; cover, and cook slowly for twenty minutes. Moisten a tablespoonful of flour in a half cup of milk; when perfectly smooth, add another half cup; turn this into the mushroom mixture; bring to boiling point, add just a grating of nutmeg, a few drops of onion juice, and a dash of pepper. Serve as you would stewed oysters.
To make this into à la poulette, add the yolks of two eggs just as you take the mixture from the fire, and serve on toast.
Trim the soft gill portion of the Pleurotus ostreatus into the shape of an oyster; dust with salt and pepper; dip in beaten egg, then in bread crumbs, and fry in smoking hot fat as you would an oyster, and serve at once. This is, perhaps, the best method of cooking this variety.