Heat the oven to 150C/130C fan/ gas 2. Heat the oil in a casserole dish over a medium heat. Season the beef and sear until browned all over, approx for 10-15 mins, then place on a plate.Add a little more oil and the onions to the pan, and cook until soft, then add the garlic and cook for a minute more, before adding in the spices and mustard powder to make a paste.
Stir in the sugar, and bubble for a few minutes before adding the ketchup and stock. Stir well and bring to a simmer then return the beef to the pan and spoon over the sauce. Cover the pan tightly with foil, then the lid, and transfer to the oven for 4 - 6 hrs (check it at 4 hrs but it can take longer to get the beef to shred).
Once the brisket is fork-tender, leave to cook for a little bit then using two forks, shred the beef into the sauce and mix well together.
Serve in the casserole dish and let everyone build their own burger.