Place all the chimichurri ingredients, apart from the oil, in a bowl and mix well. Gradually mix in the oil until reaching desired consistency. Season and taste (Onion and garlic can be fried in a little oil prior to mixing if you prefer a weaker flavour). Transfer the sauce to a blender and blitz for a minute or so to achieve a smoother finish. Taste again and add more seasoning or oil if required. Pour into a small dish to serve.
For the Rosemary potatoes:
Pre-heat oven to 180°C. Cut potatoes in half and spread on a baking tray. Drizzle with 1 tbsp olive oil, sprinkle over the rosemary and sea salt, then bake in the oven for around 30 minutes. Check potatoes regularly and turn with a spatula. Remove from the oven once golden brown. Leave in the oven on a low heat (60-70 degrees C, until ready to serve)
For the steak:
Place a large griddle pan or frying pan on a high heat. Prep the steak by drizzling both sides with olive oil and seasoning well (use fingers to rub the oil and seasoning all over the meat). Once the pan is hot place the steaks in the pan and leave. Cook on the first side for 3-4 minutes before turning over and cooking for a similar length of time on the other side (this will cook the steak rare – medium, adjust the time according to your preferences). Transfer the sealed steaks to a baking tray and place in a low oven 60-70°C to rest for 5 minutes. When ready to serve remove the steaks and slice into thin pieces, then arrange with the potatoes on the platter board.