Heat your oven to 160°C/Fan 140°C/Gas 3. Dissolve the beef stock cube in 1L boiling water.
Heat a heavy-based frying pan over a high heat. When it’s smoking hot, pour in 2 tbsp oil. Sear the brisket on all sides till browned, using tongs to turn it. Transfer the brisket, along with any oil, into a casserole dish with a lid. Set aside.
Peel and thinly slice the onions. Peel the garlic cloves. Add them to the dish with the brisket. Pour in the beef stock. Add 2 tbsp each of the tamari and Worcestershire sauce. Pop it into the oven for 4 hours, turning the brisket over a couple of times during cooking.
Remove the dish from the oven and lift out the brisket. Pop on a board. Cover with foil and leave to rest for at least 30 mins.
Taste the gravy in the dish. Add salt and pepper if you think it needs it. If you prefer a thicker gravy, whisk 2 tbsp of the gravy with 1 tbsp cornflour or plain flour. Slowly pour it back into the pot and stir. Cook over a medium heat for 10 mins till the gravy is as thick as you like.
Cut the brisket into thin slices. Serve with the onion gravy.