Combine the oil, lemon zest, lemon juice, brown sugar, pepper and sherry in a large dish. Add the beef and marinate for 2 to 3 hours.
Preheat the oven to 180°C.
Transfer the beef to a baking dish and roast to desired doneness: • about 45 minutes for rare (52°–54°C) • 1 hour for medium (60°–65°C) • 1¼ hours for well-done (65°–70°C)
Remove roast from oven, cover with foil and rest for 20 minutes to allow the juices to redistribute before slicing and serving.