It is cow pancreas. It used to be a staple at bar mitzvah buffets. You blanch them, take off the membrane, and saute with mushrooms, then make a roux and thicken it with chicken broth for the gravy. Keep it warm in a chafing dish and serve over either steamed rice or toast points. It was considered a real treat, but now I have to watch my cholesterol, so no more sweetbreads or chopped liver. Bummer.