Clean and parboil the Sweetbreads for twenty minutes. Then remove all veins and nerves and chop the meat into pieces of an inch and a half. Chop the mushrooms very fine indeed. Put the butter in a saucepan, and when it melts, add the flour, being careful not to let it brown. When perfectly smooth, add the milk and stir constantly till it boils. Add the chopped mushrooms and simmer for five minutes. Season well to taste with salt and white pepper. Add the Sweetbreads and cook for five minutes longer and serve hot. At luncheons and dinners the Sweetbreads a la Creme are served in small silver shells or fancy paper cases.