Season the flank steak lightly with salt and let it sit at room temperature for 30 minutes. Pat dry with paper towels and season again with salt and freshly ground black pepper.
Build a medium-hot fire in a charcoal grill, or heat a gas grill to high.
Place the steak on the grill and cook for 3-4 minutes per side until meat is nicely charred and medium-rare. As it cooks dot it with small knobs of unsalted butter to keep it moist and build flavour.
Transfer the steak to a carving board and let it rest for at least 8 minutes.
While the steak is resting make your chimichurri sauce. Mince the garlic (or chop as fine as you can).
Trim and halve the chilli and use a spoon to remove the seeds and roughly chop (leave the seeds in for extra heat).
Roughly chop the shallot, the coriander and the parsley.
Place all this in a bowl with the vinegar, oregano, and 1 tsp salt. Using a fork, whisk in oil until everything is nicely coated.
Slice the steak against the grain, spoon the chimichurri sauce over making sure each slice gets some and serve.
