For the chimichurri sauce, place the chopped chillies and garlic into a bowl.
Add the remaining chimichurri sauce ingredients and stir well until combined. Cover the bowl with a sheet of cling film, then set the mixture aside for 4-6 hours to allow the flavours to infuse, or preferably overnight.
One and a half hours before serving the meal, light the barbecue outdoors and leave the coals to heat through for one hour.
When the coals are white hot and the flames have died down, suspend a cooking rack 25cm/10in above the coals.
Season the beef sirloin all over with salt flakes and freshly ground black pepper, then place the seasoned beef sirloin, fatty-side down, onto the cooking rack. (NB: Ideally, during cooking, the barbecue should be covered with a lid and the vents should be open.)
Sear the fatty side of the sirloin for 4-6 minutes, or until golden-brown, then turn the beef over and continue to cook for 20-25 minutes.
Turn the sirloin steak over again and continue to cook for a further 20 minutes (for medium), or until the sirloin is barbecued to your liking.
When the beef sirloin is cooked to your liking, remove it from the barbecue and set aside to rest for ten minutes, before carving into thick slices.
To serve, arrange 2-3 slices of the barbecued beef sirloin onto serving plates and spoon over the chimicurri sauce.