Heat the ghee or fat in a frying pan over a high heat until smoking.
Season the steaks on both sides and fry in the hot pan until cooked to your liking (they will continue to cook after you take them off the heat). (For a 2cm thick steak this will be about 11/2 mins on each side or 2 mins for medium. Don’t touch them until they are ready to turn over.)
Transfer the steaks to a plate, cover with another plate or bowl and rest for 5 minutes to allow the juices to relax back into the the meat.
In the same pan, sweat the leek or onion in the meat juices over a low heat for 3 minutes until soft.
Stir in the mustard and season, then add a splash of wine, broth or water if you want a thinner sauce.
Serve the steak with the sauce over the top and your choice of vegetables or salad.