Take the joint out of the fridge 2 hours before you want to cook it.
Preheat the oven to 220C (fan). Slice the onion into quarters and put it in the middle of the tray. Top with the garlic cloves and herbs then put the rib of beef on top and season well with salt and pepper.
Cook the rib roast for 20 minutes then turn down the heat to 160C, and cook for another hour, until the internal temperature reaches 40C (it will rise as it rests to around 55C). Remove from the oven and transfer the meat to a clean tray. Cover tightly with 2 layers of foil and a clean tea towel and leave to rest for 1 hour. Save the cooking tray to make the gravy.