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Heat 1 tbsp oil in a large, deep roasting tin (one that can go on the hob) or in your largest flameproof casserole dish. Season the beef well and sear in the tin until nicely browned all over. Lift out and put on a plate or board. Add another 1 tbsp oil and the onions to the tin and sweat for 8-10 mins until really soft. Meanwhile, in a small bowl, mix the paprika, mustard powder, cinnamon, cayenne or chilli, treacle, 2 tbsp vinegar, 2 tbsp sugar and the remaining oil. Brush all over the meat, reserving about 2 tbsp for later. Heat oven to 150C/130C fan/gas 2.
Add the garlic, bay leaves and chillies to the softened onions, stir around the pan for 1-2 mins, then pour in the passata, Worcestershire sauce, remaining vinegar and sugar. Season the sauce, give a good stir, then put the brisket on top. Wrap the tin tightly in a few sheets of foil, or cover with a lid, and bake for 6 hrs, turning once or twice during cooking, until really tender.
Uncover the dish, give the sauce around the meat a little stir and brush the remaining spice paste over the meat. Can be made a day ahead up to this point – cool then chill the meat and sauce; remove from the fridge and scoop off any fat from the sauce before reheating. Increase the temperature to 200C/180C fan/gas 6. Cook for a further 20 mins (or 40 mins from cold, covered for 20 mins) until the meat is dark and sticky. Remove from the oven, cover loosely with foil and leave to rest for 15 mins.
To serve, skim any fat off the surface of the sauce, shred the meat with 2 forks, discarding any butcher’s string as you go, and toss the meat through the sauce, discarding the bay leaves and chillies. Pile into warm flatbreads and serve with Charred sweetcorn salsa (see 'Goes well with') and a dollop of soured cream.