Preheat the oven to 240°C/475°F/ gas 9. Take the beef out of the fridge 30 minutes before you want to cook it and allow it to come up to room temperature. Place a large sturdy roasting tray in the oven to heat up. In a pestle and mortar smash 3 garlic cloves and most of the the rosemary leaves with a pinch of sea salt and a lug of olive oil, then massage all over the beef.
Quickly bash the remaining unpeeled garlic cloves and add to the hot roasting tray with the beef. Pop straight in the oven and roast for around 50 minutes, basting occasionally with the juices from the tray.
When the time’s up, reduce the temperature to 190°C/375°F/gas 5 for around 10 minutes, or until the beef is beautifully golden brown on the outside and pink in the middle – leave in for longer if you prefer your beef well done. Carefully transfer it to a platter, dot the knob of butter on top, then use a rosemary sprig to brush all over the meat. Cover with a double layer of tin foil and a tea towel and leave to rest.
Serve the beef with the resting juices drizzled over the top, perfectly golden roast potatoes, Yorkshire puds, dressed watercress and a good smear of horseradish.