1. Preheat the barbecue until it is very hot. Season the steak on both sides.
2. Cook the steak, flipping it every 15 seconds. A 2cm steak will take about 2 minutes to reach medium-rare.
3. Towards the end of cooking, use a digital probe to monitor how close the meat is to readiness. (You can see below the temperatures at which meat is bleu, rare, medium-rare, etc.) Insert the probe into the thickest part of the flesh and make sure you don’t push right through to the base of the pan. You’ll need to take the meat out of the pan about 5 degrees before it reaches the exact temperature you want: the steak will carry on cooking after it’s removed.
4. Rest the meat for 5 minutes before serving. This allows residual heat to finish the cooking, and lets the fibres relax, so that they hold on to more of their juices. Do this resting on a rack set over a plate: if you place the steak on to a hard flat surface, the heat can’t escape from the underside and so might overcook it.
5. While the steak is resting, make the sauces. For the chimichurri, combine all of the ingredients in the jug of a blender and blitz until smooth. Place in an airtight container and place in the fridge until needed. For the guasacaca, combine all the ingredients together in the jug of a blender or food processor and blitz until smooth. Transfer to an airtight container until needed.
6. Cut the steak into slices if you want, and season with salt and freshly ground black pepper. Serve with the South American sauces.
45ºC (bleu), 50ºC (rare), 55ºC (medium-rare), 60ºC (medium) and 70ºC (well done).