Cut the fillet into four medallions. Finely chop the shallot and oregano, mix with the mustard. Coat the medallions in the mixture, cover and leave to marinate in the fridge for approx. 30 mins.
Heat the clarified butter in a frying pan, fry the medallions for approx. 3 mins. on each side, remove from the pan and keep warm.
Wipe the cooking fat from the pan, pour in the sherry, loosen any bits that have stuck to the bottom, reduce a little. Add the stock, bring to the boil, reduce the liquid to approx. 50 ml. Remove the pan from the heat and gradually add the butter, stirring with a whisk. Return the pan to the hob a few times to barely warm the sauce. Serve the meat and sauce together.