Cut the meat with a very sharp knife on the fat layer in a diamond shape. But the butcher from whom you buy it does the same. Rub the roast beef all around with salt and colored pepper crushed in a mortar and brush on with several dashes of Worcester sauce.
Melt a good amount of clarified butter in a large pan (do not use a Teflon pan, otherwise the fat will burn) and let it get very hot. Sear the roast beef on each side for about 5 minutes, always brushing the seared sides with Worcester sauce. Then place the roast beef on the grid in the middle of the oven and cook for at least 3 hours at 80 ° C.
The roast beef is ready to eat after 3 hours. But it can also stay in the oven for 4 hours and is then even more tender.
This goes well with sauce Bernaise and baguette or rosemary potatoes and a dry red wine.
With this preparation it does not matter whether you take 2 or 3 kg of roast beef and whether the guests arrive on time. Even if you are an hour late, the meat is still perfect thanks to the low-cooking method and has wonderful roasted aromas when seared.