1. Preheat the oven until 200°C.
2. Rub the duck fat all over the steak.
3. Season the steaks with salt to taste.
4. Heat a cast iron pan until smoking. Place the steak in pan. Cook for 2 minutes undisturbed to ensure the pan is properly sealed.
5. Turn and cook the other side for 3 minutes. Finish it in the oven at 180°C for 15 minutes for medium rare. Allow the steak to rest for 10 minutes, loosely covered with foil.
6. Slice the meat down from the bone into slices with a sharp knife.

To make the mushroom and green peppercorn sauce:

1. Heat the butter and olive oil in a pan over a medium heat.
2. Add the mushrooms and cook undisturbed for 1 minute to ensure they caramelise without sweating.
3. Add the garlic and toss to coat the mushrooms, then add the cream.
4. Cook for a further minute, then add the green peppercorns and season to taste. Serve the sauce alongside the steak.