Preheat the oven to 240C/475F/Gas 9.
Place the beef, just as it is, upright in a roasting tin, tucking in the half onion alongside it.
Combine the mustard powder and flour, then dust this all over the surface of the fat, and finally season with a few twists of freshly milled pepper. This floury surface will help to make the fat very crusty (for those like me who want to eat what I call the crispies), while the onion will caramelise to give the gravy a rich colour and flavour.
Place the joint in the oven - it will have plenty of fat so do not add extra. After 20 minutes turn the heat down to 190C/375F/Gas 5, and continue to cook for 15 minutes per lb (450 g) for rare, plus 15 minutes extra for medium-rare or 30 minutes extra for well-done.
While cooking, baste the meat with the juices at least three times. To see if the beef is cooked to your liking insert a thin skewer and press out some juices: the red, pink or clear colour will indicate to what stage the beef has cooked.
Remove the cooked beef to a board for carving and leave it to rest for at least 30 minutes before serving (while it is resting you can increase the heat in the oven to finish the roast potatoes if you are serving them).
This resting period allows most of the juices which have bubbled up to the surface of the meat to seep back into it, and the meat itself firms up to make it easier to carve. Some of the juices will escape, though, and these should be poured into the gravy.