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.