Take the beef out of the fridge 1 hour before you want to cook it, to let it come up to room temperature. Place a large sturdy roasting tray in the oven to heat up.
Strip most of the rosemary leaves into a pestle and mortar, and peel and add 3 cloves of garlic, then bash to a paste with a pinch of sea salt and a lug of oil.
Tip the paste onto the beef and massage all over, getting into all the nooks and crannies.
Quickly squash the remaining unpeeled garlic cloves and scatter into the hot roasting tray with the beef. Place straight into 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 the beef to a platter, dot the knob of butter on top, then use the remaining rosemary to brush all over the meat. Cover with a double layer of tin foil and a tea towel, then leave to rest.
Serve the beef with the resting juices drizzled over the top – delicious served with perfectly golden roast potatoes, Amazing Yorkies, dressed watercress and a good dollop of horseradish.