Score the fat layer of the roast beef in a diamond shape. Mix the ingredients into a marinade and leave the meat to marinate in it for one night in the refrigerator. Roast in a heated stove at 200 °C for about 1 hour, always basting with marinade. Boil the remaining marinade with beef stock in a saucepan and thicken with flour. Arrange meat on a platter, pour sauce over it and garnish with rosemary.
Serve with dumplings or boiled potatoes.