Trim fillet of all fat and gristle. Season with pepper.
Heat butter and oil over high heat in heavy frying pan.
Sear gillet and brown on all sides.
Retain pan juices.
Place fillet in a roasting pan and cook in a preheated oven at 200°C for 15-20 minutes (or until cooked as desired).
When cooked remove from roasting pan and stand for 5 minutes.
Meanwhile, combine all meat juices in heavy frying pan.
Over a medium high heat cook onion in pan juices until soft.
Add peppercorns, mustard, brandy and water and allow to simmer quickly until thick and reduced.
Remove from heat and slowly add in yogurt.
If necessary, reheat sauce without boiling.
Place meat on serving dish and our sauce over.
Ideal fat for frying is a mixture of half butter and half oil.
Butter for flavour and oil to prevent butter burning and becoming bitter.