Pop the beef into an oven dish on a roasting rack. Salt the rib of beef all over, get it properly coated, then crack the black pepper all over.
Mix the mustard, rosemary, thyme, and garlic in a bowl. Rub this all over the beef and let it sit at room temp for 30 minutes. You can wrap each bone in foil if you want to prevent them from charring.
Preheat the oven to 120C. Pop the beef on the middle shelf and cook until the internal temp hits 50C, around 2 1/2 to 3 hours depending on your oven.
Pull it out and rest it for 30 minutes under tin foil.
Turn the oven up to 260C, or the highest it goes. Put the beef back in for 10 minutes to get that proper crust.
Take it out, leave it to stand for at least 5 minutes, and then slice the meat off the bones, then carve into portions.