Bring Beef to room temp: Take beef out of the fridge 2 – 3 hours before cooking to bring to room temp (key tip for even cooking). Pat dry with paper towel.
Preheat oven to 240°C/460°F (220°C fan). Adjust shelf so beef will be sitting in the middle of the oven.
Garlic Herb Butter: Mix together.
Roasting bed: Place onion, garlic and herbs in a heavy based oven proof skillet (or use a roasting pan).
Slather 1: Spread a thin layer of butter on the underside of the beef (ie the bone side). Place beef on onion etc, butter side down. Spread about 2/3 of the butter on the top and sides (reserve some for Slather 2).
Hot oven: Roast 20 minutes.
Slather 2: Remove, spread over remaining butter. Turn oven down to 120°C/250°F (100°C fan).
Slow roast: Roast for a further 1 1/2 hours, basting every 30 minutes with the juices in the pan, until the internal temperature is 50°C/122°F in the centre (for medium rare, Note 4). Start checking the internal temp early.
Rest: Transfer beef to plate. Cover loosely with foil and rest for 20 – 30 minutes. Internal temperature will rise to 54°C/129°F (which is medium rare).
Slice beef and serve with Sauce! For a classic high-end Steakhouse experience, serve with Paris Mash and Garlic Sautéed Spinach.