Get some little cases from the pastry-cook of puff paste, which are to be filled with sweetbread cut in dice. It is a good plan to heat the cases before filling them.
Cook the sweetbreads in water with pepper and salt, till done, skin them and cut in dice. Prepare a good bechamel sauce, seasoned with the juice of a lemon, and add to it a few mushrooms that have been fried in butter. Heat the dice of sweetbread in this sauce and fill the cases with it. Put them back in the oven to get quite hot.