Weigh the beef to establish the cooking time. Preheat the oven to 220C/425F/Gas 7.
Crumble 1 Beef OXO cube into a small bowl, mix with the flour and mustard powder. Dust this over the fat surface of the beef, rubbing in gently. This will become nicely crusty during cooking as well as adding flavour. Now season with freshly ground black pepper.
Place the beef in a roasting tin and tuck the onion quarters around the meat. Cook for 20 minutes then reduce the oven to 160C/325F/Gas 3 and cook slowly for 20 minutes per 450g for medium rare and 15 minutes per 450g for rare.
Whilst the meat is cooking, baste it in its own juices to keep it moist and succulent. Transfer the cooked meat to a carving plate and loosely cover with foil to rest for 10–15 minutes.
Meanwhile to make the gravy: tilt the roasting tin and spoon off all but 2 tbsp of the fat, also retaining the brown juices and the caramelised onion. Place the mixture in a saucepan over a gentle heat and allow the juices to sizzle then sprinkle in the flour and blend to a smooth paste. Let the flour cook for a moment or two until browned. This helps to produce a rich colour gravy.
Crumble and stir 1 Beef OXO cube into 600ml (1pt) hot water.
Using a wooden spoon, loosen the tasty crisp brown bits stuck to the tin whilst gradually adding the hot stock. Stir briskly until the simmering gravy is smooth and thickens. If you prefer your gravy thicker, simply leave to bubble and reduce for a minute or two.
Carve the beef onto warm plates, accompanied with roasted potatoes and Yorkshire puddings and enjoy with lashings of hot gravy.
For rich red wine gravy, replace 150ml (¼ pt) water with a robust red wine.