Answer:
   
   The problem with Filet Minion is Gid Hanasheh, the sciatic nerve. The
   Torah prohibits eating of the sciatic nerve in both hind thighs of any
   kosher land animal (domesticated or wild). The difficult process of
   the removal of the nerve and the fat surrounding it. This is called
   "nikur", and must be done with great care by a skilled expert. In most
   countries, the difficult process of removing the prohibited fats and
   nerves is avoided entirely by not eating (at all) the hind part of an
   animal. This also makes it difficult to find Kosher filet mignon, rump
   and sirloin steaks, leg of lamb, and London broil.

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