Sweetbreads are very perishable and should be soaked and pre-cooked in advance, particularly if not used immediately.
Soak the sweetbreads in cold water for 2 hours to clean them. Cut away any discolored parts. Blanch sweetbreads by putting them in cold water, bringing to a boil and simmering for 5 minutes (calf’s) or 3 minutes (lamb’s), then drain them.
Pull away the ducts. Remove the skin and outer pieces of membrane from the sweetbreads.