First make the chimichurri. Put the parsley and coriander into a bowl with the oregano. Add the chilli, ginger and garlic then pour in the oil, vinegar and soy. Stir to combine, taste and add fine sea salt if needed.
Preheat a large frying pan or saute pan over a high heat until it is extremely hot. Meanwhile drizzle the steak on both sides with olive oil and season well with salt.
Add the steak to the hot pan and reduce the heat to medium high. Fry for 3 minutes on each side, adding butter when you flip the steak and scooping the melted butter up over the steak. Now hold the steak with tongs and press the fat against the pan for a minute to render and crisp if you are eating the fat.
Put the steak onto a chopping board and rest for a couple of minutes. Add the pak choi to the frying pan and fry for a minute or two on each side until it is charred and the dark green leaves have just wilted.
Slice the steak and serve with the pak choi and well coated with the chimichurri.