To cut the cauliflower steaks, remove the leaves from the base of the cauliflower and cut the cauliflower into 4 steaks that are each 1/2 inch thick, being sure to get a portion of the core in each one as this will hold them together.
In a food processor combine all of the ingredients for the chimichurri and blitz until smooth.
Brush the top side of each cauliflower with a spoonful of the chimichurri sauce and then place the steaks oiled side down on the grill. Close the lid to the barbecue and let the steaks cook for 5 minutes. Brush the top sides of each steak with chimichurri sauce before flipping them over and grilling for another 5 minutes. The steaks should be tender with darkened grill marks on both sides.
Serve the cauliflower immediately with the chimichurri evenly spread on top.