Mushroom Soufflé

From 1922:

Boil some mushrooms in milk, the quantity to vary with the size of the soufflé basin. Make a cream sauce with flour, milk, cream and butter; add the mushrooms which have been chopped in dice. Beat up the yolk of 2 eggs (for three persons); add pepper, salt and a squeeze of lemon juice. Put out the soufflé basin, whisk the white of the eggs to a snow, lightly stir in and steam for ¾ of an hour. Serve with a table napkin round the basin and sprinkle chopped parsley on the top of the soufflé.