Clean the mushrooms and cut them into pieces. Put them in 3 cups boiling salted water, to which you've added the cup of wine, salt and black pepper. Boil everything for half an hour, after which drain the broth from the mushrooms.
Separately, fry the flour with the butter and dilute with the mushroom broth, then add the mushrooms themselves. Add the nutmeg and allow the sauce to simmer for about 10 minutes.
If it becomes too thick, dilute with a little hot water. Serve with steaks and a garnish of French fries.