Pre-heat the oven to 240C/475F/Gas 9.
If you dust the fat surface of the beef with mustard and the flour - just rub them in gently - then season with salt and pepper, it becomes extra crusty during cooking. So do that first, then place the joint in the roasting tin and tuck the two pieces of onion in close to the meat. The onion will caramelise as the beef cooks and give a lovely flavour to the gravy.
Now place the meat just above the centre in the oven and give it 20 minutes cooking at the initial temperature; after that turn the heat down to 190C/375F/Gas 5 and cook it for 15 minutes to the pound 450g/16oz for rare, adding another 15 minutes for medium rare and another 30 minutes for well done. While the beef is cooking, lift it out of the oven from time to time, tilt the tin and baste the meat really well with its own juices - this ensures that the flavour that is concentrated in the fat keeps permeating the meat, and at the same time the fat keeps everything moist and succulent. While you're basting close the oven door in order not to lose heat.
When the beef is cooked, remove it from the oven, transfer it to a board and allow it to stand in a warm place for up to an hour, loosely covered with foil, before carving - to let all the precious juices that have bubbled up to the surface seep back into the flesh. Also, as the meat relaxes it will be easier to carve. Meanwhile, make the gravy.
After removing the meat from the roasting tin, tilt to see how much fat remains - you need about 2 tbsp for this amount of gravy (the rest can be spooned into a dish and used for Yorkshire pudding). Place the roasting tin over a medium heat and sprinkle the flour into the fatty juices. Then, using a wire whisk, blend in the flour using a circular movement.
When you have a smooth paste, slowly add the hot vegetable water, whisking all the time, and scraping the base of the tin to incorporate all the residue from the roast. When the gravy is bubbling, taste to see if it needs a little more seasoning, then let it carry on bubbling and reduce slightly to concentrate the flavour.
You can now pour the gravy into the jug and keep it warm if lunch is imminent or, if not, leave it in the roasting tin and re-heat gently just before serving.
To make the horseradish sauce, simply mix all the ingredients together in the bowl you're going to serve it in.