To make the chimichurri sauce, first peel the garlic and deseed the chillies. Chop both very finely and scoop into a clean jam jar. Remove the tough stalks from the parsley and finely chop the leaves. Add the parsley leaves to the jar along with the oregano, salt, vinegar, oil and water. Stir until all the ingredients are well combined. Put the lid on and leave to sit for several hours or overnight, allowing the flavours to develop.
Roughly 1½ hours before serving, light the barbecue, making sure that you use enough coals to maintain a good temperature for at least an hour. Season the beef all over with salt and a few twists of black pepper.
When the coals are dusted with a fine coating of grey ash and all the flames have gone (this is essential, as otherwise the meat will burn) – after about 30 minutes – put the beef on a rack over the top so that the meat is around 25cm from the coals. Sear on the fatty side until well coloured, then cook fat-side up for 25 minutes. Turn the sirloin again before cooking for a further 20 minutes. Ideally, the barbecue lid should be on and the vents open. When the beef is done, lift onto a board and leave to rest for 10 minutes.
Carve the beef into thick slices. Give the sauce a good shake and spoon it over the beef as it is served. Fried potatoes with a tomato and onion salad would be spot-on.
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