Place the tied brisket in a large roasting tin, tucking the garlic and thyme around and under the meat. Massage the oil into the brisket, then season generously with salt and pepper. Place it in an oven preheated to 200°C/Gas Mark 6 and roast for 30 minutes, then remove from the oven.
Reduce the oven temperature to 140°C/Gas Mark 1. Cover the meat with foil, return it to the oven, and roast for 4 hours, or until very tender.
Baste the meat with its juices, then add the potatoes and onions to the roasting tin, tossing them in the juices as well. Increase the oven temperature to 160°C/Gas Mark 3 and cook, uncovered, for another hour, until the brisket is crisped up and the potatoes and onions are fully cooked.
Serve the beef in thick slices alongside the potatoes and onions. There shouldn’t be much liquid left, as the vegetables will have absorbed most of the juices, but any remaining should be spooned over the meat.
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