Soak 2 lbs of sweetbreads in water for two hours, then cook for about 15 minutes in boiling water. Remove, rinse in cold water, discard the membrane, and lard all over with ¼ lb finely cut up pork fat. Meanwhile, melt 1½ spoons butter in a stewpan, add 1 grated onion, stir in ½ spoon flour, and fry lightly. Dilute with 1 glass bouillon and add ½ cup grated cheese, a little salt, pepper, nutmeg, and 1 glass cream. Add the sweetbreads, cover, and cook until tender. Transfer the sweetbreads to a platter and cover with the sauce. If desired, a handful of picked over and finely chopped field mushrooms may be added to the sauce with cleaned crayfish tails and 1 spoon crayfish butter.