Preheat the oven to 220'C.
Trim any unwanted sinew from the joint. Rub the seasoning all over the beef with the olive oil.
Peel the onions and slice into thick rings. Cut the garlic bulbs in half. Place the onion rings, thyme and garlic on a roasting tray. Then place the joint on top and roast in the oven for 10 minutes. Turn the temperature of the oven down to 180'C and continue to roast until the core temperature reaches 48'C (medium rare). This should take around 25-30 minutes. Rest the meat for at least half an hour to allow the juices inside to relax.
Before serving, heat in the oven for 3 minutes.
«Make sure your joint is at room temperature before roasting.»