Place finely chopped chillies and garlic into a bowl.
Stir in remaining chimichurri ingredients and let infuse for 4-6 hours or overnight.
Light the barbecue coals and allow them to heat for one hour.
Position a cooking rack approximately 25cm above the white-hot coals.
Season the beef with salt and pepper and place it fat-side down on the rack.
Sear the fatty side for 5 minutes, then flip and cook for 22 minutes.
Flip the steak again and cook for an additional 20 minutes for medium doneness.
Remove the beef from the heat and rest for 10 minutes before slicing.
Serve thick slices of beef drizzled with the prepared chimichurri sauce.