Preheat the oven to 240°C.
Place the beef upright in a roasting tin and tuck the onion alongside it.
Dust the combined mustard powder and flour over the fat and season with pepper.
Roast the beef at 240°C for 20 minutes.
Reduce heat to 190°C and cook for 33 minutes per kg for rare, plus extra for medium.
Baste the meat with roasting juices at least three times during cooking.
Test doneness by inserting a skewer to check the colour of the juices.
Remove the beef to a board and rest for at least 30 minutes before carving.
Collect any escaped juices to add to the gravy.