Pull your steak out of the fridge and let it sit and come to room temperature. This is important. Do this! While the steak is doing its thing, start with the green sauce.
First, char the Broccolini. Whack a griddle pan over a high heat, once it starts to smoke, dress the broccolini with salt, pepper and olive oil, throw into the pan and char for 1 minute each side. Don’t be scared of a little colour.
Once the broccolini is cooked and cooled a little, cut an inch off each stem. Finely slice these end bits, whack in a pestle and mortar. Finely chop the parsley and add to the mix. Add a pinch of salt and grind the broccolini and parsley together. Once combined, add dijon mustard, a tablespoon of olive oil, a good crack of black pepper and mix again with the pestle. It will start to thicken to a sauce. Finally, add a tablespoon of your diced red onion, mix through - you have your green sauce!
Now it’s game time with the steak. Sprinkle with salt and olive oil and place on the searing hot griddle pan. Depending on the thickness of the steak, let it cook for 2 minutes on one side, before turning and continuing to cook for a minute and a half on the other. Remove from the pan and let it rest. The old rule of thumb is to let the meat rest for half the total cooking time...in this case 1 to 2 minutes tops.
To serve, slice the steak and arrange on the plate, top it with the broccolini and drizzle the green sauce on top.