Parboil and blanch the sweetbreads, and let them get cold. Cut into small dice of uniform size. Make a white roux of one tablespoonful of butter and the same of flour, and stir into it a cupful of hot milk, continuing to stir until it bubbles all over ; add now a cupful of chopped mushrooms, the sweetbread dice, a little chopped parsley, pepper and salt to taste, and set the saucepan in boiling water for five minutes before serving.