Soak the dried mushrooms in the lukewarm water for 20 minutes, then drain, but save the water. Rinse mushrooms, removing sand attached to the stems. Place the mushrooms on a board and chop them coarsely. Set aside.

Strain mushroom water several times through paper towels to remove sand, then pour into a heavy enameled saucepan. Use this water as the basis for the sauce. Heat until the water is close to the boiling point. Put the butter in another heavy saucepan and place it over low heat. When the butter is melted and quite hot, add the flour and stir with a wooden spoon. When flour is all incorporated and begins to sizzle, pour all the hot mushroom water in at one time. Stir continuously for 3 to 4 minutes, to prevent lumps from forming.

Add the chopped mushrooms; taste for salt and pepper. Simmer for 12 to 15 minutes more, stirring every so often, then transfer to a sauceboat and serve hot.