Thinly slice the garlic cloves. With the tip of a knife, cut slits all over the roast beef and insert the garlic slices.
Combine the thyme, rosemary, oil, pepper, and a pinch of sea salt in a mortar and pestle. Pound to a purée and mix with the mustard. Rub the mixture all over the beef rib.
Place the beef on a rack in a roasting tray. Preheat your oven to 160ºC and roast for about 2 hrs until the fat has started to go golden. I suggest the cooking time should be 20 mins per lb plus 20 mins extra.
Transfer the roast to a cutting board. Cover with foil (shiny side down) and leave to rest for 10–15 mins before cutting. Reserve juices for gravy.
Roast potatoes
When the oven is hot, add oil to a baking dish and place on the bottom rack to heat. Once hot (about 3 mins), add dry potatoes, rosemary, and garlic. Coat in oil and season generously with salt and freshly ground black pepper.
Return to the oven on the bottom rack, and roast, checking occasionally so that it doesn’t burn, until potatoes are crusty and dark golden brown, about 35 mins. Remove the rosemary, stir to coat potatoes with oil and break up garlic cloves, season as necessary and serve.
