Sirloin with Chimichurri Sauce

I have splashed, daubed, painted and dobbed all sorts of sauces, dips, marinades, cures and unctions over the noble beef sirloin, but the discovery of chimichurri was a true revelation - and no wonder, it coming from a land whose currency is beef. I have never travelled in Argentina, but with some research, foreign calls and good advice I feel that this recipe can rightfully call itself chimichurri and stand as proud as the gauchos of the Pampas.