STEP 1
Heat the oven to 170C/fan 150C/gas 3. Heat 2 tbsp of the oil in a casserole over a medium-high heat. Season the brisket all over and brown really well on all sides, then remove onto a plate. Turn the heat down and add the remaining 2 tbsp of oil, sliced onions and a big pinch of salt. Put the lid on and cook, stirring regularly, for 20-30 minutes or until soft. Tip in the flour, mustard powder and thyme, and mix well. Cook for 2 minutes, scraping any crusty bits off the bottom of the pan.
STEP 2
Add the Bovril, worcestershire sauce, the brisket and enough beef stock to come halfway up the beef joint. Cover tightly with foil, put on the lid and cook for 3 hours, then remove the lid and cook until the beef is meltingly tender and the sauce thickened.