Heat oven to 200C/180C fan/gas 6. Toss the potatoes with half the oil, the garlic, rosemary and some seasoning. Arrange in a single layer on a baking tray and cook for 45 mins, turning halfway through, until golden and crisp.
About 10 mins before the potatoes are done, heat the remaining oil in a large pan on a high heat. Season the steak with salt and lots of black pepper and cook for 2-3 mins each side. Transfer to a plate and cover with foil to rest while you make the sauce.
Tip in the vinegar and bubble for a few seconds, then pour in the stock. Reduce by half, then add the cream, some salt if needed, and lots of black pepper. Cook for 1-2 mins until thick and creamy. Serve the steaks in chunky slices topped with sauce along with the potatoes, with some green beans, if you like.