Prepare your beef by cleaning it under some cool running water. Pat dry with some paper towels.
Prepare the chimichurri salsa by combining all the ingredients (except the olive oil) in a food processor. Whizz everything together and begin pouring the olive oil through the spout in a steady stream until everything comes together and you have some kind of paste/sauce. I like to add a little extra olive oil and keep the sauce slightly runny.
Place the beef in a non reactive bowl and pour half the chimichurri over it. Massage the marinade into the meat. Cover and refrigerate for at least four hours or overnight.
Allow the beef to come to room temperature (approx one hour before roasting) and place in a hot 180 deg C oven for half an hour. (I like to drizzle a little more oil over the beef before I roast it)
Turn the temperature down to 160 deg C and turn the beef over and baste it with combination of juices and marinade-do this every half hour (if you find it’s a little dry add a little water)
After approx 2 hours take your beef out and allow it to rest, covered with aluminium foil.