Mix together all ingredients in a bowl. Place the eye fillet in a large container or roasting dish. Pour the marinade all over the beef and rub it in with your hands. Cover and let it marinate for at least 2 hours, but overnight if possible.
Take the beef out of the fridge 20 minutes before cooking. Preheat a covered barbecue on high. Cook on the barbecue flat plate for 5 minutes, turning often, until browned all over.
Transfer to a large baking tray. Reduce the heat to low. Cook, with the barbecue lid down, for 20 minutes for medium-rare or 25 minutes for medium. Remove from the barbecue and transfer the beef to a plate. Cover loosely with foil and set aside to rest for 15 minutes.
Serve the eye fillet with chimichurri (see recipe below)
Chimichurri Sauce
Combine all ingredients except the herbs and the olive oil in a food processor. Pulse until semi broken down but not fully broken down. Add the herbs and pulse again. After everything has been broken down, stir through the olive oil. Season well. Add a little more brown sugar if needed.
