Heat the oil in a large pan. Dust the brisket with the flour and season well with salt and pepper. Put all the chopped vegetables in the bottom of your slow cooker and turn it on to Low. Sear the beef all over in the hot pan then place it on top of the vegetables in the slow cooker. Add the bay leaves, garlic and mustard then pour over the stock. Cover with the lid and cook for 7 hours.
Heat oven to 200C/180C fan/gas 6. Carefully take the beef out of the slow cooker and place it on a baking tray then roast it in the oven for 20mins.
While the beef is in the oven, carefully ladle the cooking liquid out of the slow cooker into a shallow pan. Boil rapidly on a high heat to reduce to a rich gravy. Serve the beef sliced with roast potatoes, the softened vegetables, gravy and wilted greens, if you like.