I use a plastic water spritzer to spray the potatoes with the homemade flavoured vinegar – it gives those crispy potatoes an added smack of flavour.
Around 1 hour 30 minutes before you’re ready to start, remove the meat from the fridge and leave aside to come up to room temperature.
Preheat the oven to 200ºC/400ºF/gas 6. Place a large roasting tray in the oven to preheat. Meanwhile, slice the potatoes into rough 5cm chunks, then parboil in a pan of salted boiling water for around 12 minutes. Drain and return to the pan, shake to chuff them up, then leave to steam dry. Carefully remove the tray from the oven, add a good splash of olive oil, the butter and rosemary sprigs. Separate and bash the garlic cloves, then scatter into the tray of potatoes. Gently toss to coat, then season with salt and place in the oven for 45 minutes to 1 hour, or until golden and crisp.
Meanwhile, add the flavoured vinegar ingredients to a small plastic water spritzer, then set aside for later.
Make a herb brush by tying the woody herbs to the end of a wooden spoon with string. Preheat an ovenproof frying pan over a high heat to get it nice and hot – you want to do this around 25 minutes before the potatoes are ready. Add a splash of olive oil, then season the steak well and add to the pan, fat side-down, for a few minutes so the fat renders out. Sear the steak for around 8 minutes, or until browned all over, turning and basting with the herb brush every minute or so, then place in the oven for around 15 minutes for medium, or cook to your liking, turning halfway.
Transfer to a board, drizzle with the juices from the pan and leave to rest for around 3 minutes. When the roast potatoes are a few minutes away from being ready, spritz with the flavoured vinegar – don’t add too much, you want it to be subtle. Return to the oven for 30 seconds, then lightly squash with a fish slice. Spritz with a little more flavoured vinegar and return to the oven for a final few minutes.
Slice up the steak on a board and sprinkle with a little salt. Add a little extra virgin olive oil to the resting juices, then drizzle over the steak and serve with a nice salad, the vinegary roasties and a dollop of mustard, if you like.