Take the joint out of the fridge 30 minutes before cooking to bring it up to room temperature. Preheat the oven to 220°C, fan 200°C, gas 7. Weigh the joint and calculate the cooking time. Allow 15 minutes per 500g, plus 15 minutes for rare; 18 minutes per 500g, plus 20 minutes for medium and 22–27 minutes per 500g, plus 25 minutes for well done. If your joint is wrapped in a net to hold its shape, remove and discard then tie at regular intervals with kitchen string. Put the joint in a roasting tin, fat-side up.
Smear the mustard over the fat and season with a little salt and freshly ground black pepper – the mustard helps to form a tasty crust on the beef, but the flavour will mellow during roasting. Roast for 30 minutes, then reduce the oven temperature to 180°C, fan 160°C, gas 4 for the remaining calculated cooking time. Cover the joint with foil during roasting if the surface starts to brown too quickly.
Baste the joint every 30 minutes using the juices released from the meat in the bottom of the tin. To test the beef is ready, use a meat thermometer. Push into the centre of the thickest part of the meat: for rare beef, the meat thermometer should read 60°C, for medium, it should read 70°C and for well-done beef, it should read 80°C. If you don’t have a metal thermometer, a metal skewer works well. Insert into the centre of the meat and leave for 30 seconds. Pull it out and touch the tip against the inside of your wrist. If it is cold, the meat is not yet done. When it’s warm, the meat is rare, when fairly hot, it’s medium and when it’s very hot, the beef is well done.
When the roast is done, leave to rest covered in the foil for 20 minutes while you make the gravy. Spoon any excess fat out of the roasting tin then put on the hob over a medium heat. Sprinkle over the flour. Cook, stirring, for about 1 min. Gradually stir in the beef stock and stout (if using) and bring to the boil, stirring and scraping all the meaty residues from the bottom of the roasting tin. Let it bubble for a minute or so then strain and serve alongside the beef.