1. Preheat the slow cooker on the high setting, according to the manufacturer’s instructions, or preheat the oven.
2. Rub the beef brisket with the vegetable oil, then season with the salt and some black pepper. Tip the stock pot into a jug or crumble in the stock cube, add the boiling water and stir to combine.
3. Heat a large frying pan over a medium heat, then add the beef brisket and fry on each side for 2 minutes until browned. Place into the cooking pot of the slow cooker.
4. Add the stock to the frying pan and swirl around to combine all the juicy beef bits left in the pan with the stock. Pour over the beef in the slow cooker, then add the Worcestershire and brown sauces, stir again and cover with the lid.
5. Leave the setting on high or turn it down to low, then leave the beef to cook on high for 6–8 hours, or on low for 8–10 hours.
6. When the beef is nearly cooked, make the root veg mash. Put all the chopped veg into a large saucepan and cover with water. Cover with a lid, bring to the boil, then reduce the heat and simmer for 20 minutes until tender. Drain, then return to the pan and heat for 1 minute, shaking the pan to drive off any excess moisture. Remove from the heat, add the butter and plenty of black pepper, then mash with a potato masher until smooth. Stir in half of the chives.
7. Once the beef is cooked, remove the lid and lift the beef out onto a board. Pull it apart into pieces using two forks, then return the meat to the slow cooker to mix with the sauce/juices. Spoon the shredded beef onto plates and serve with the root veg mash alongside. Scatter over the remaining chives to finish.