Rub the beef with 1 tbsp oil, rosemary and
garlic, then leave in the fridge for at least 1 hr,
or up to 24 hrs. Make sure the beef returns to
room temperature before you cook it.
Heat oven to 200C/fan 180C/gas 6. Heat
a non-stick, ovenproof frying pan or roasting
tin until very hot. Rub most of the garlic and
rosemary from the beef, season with black
pepper and flaky salt, then sear on all sides
until dark brown and almost crusted – about
5 mins in total. Transfer the pan to the oven,
then roast the beef for 10 mins for mediumrare.
Leave to rest for a good 15 mins and
keep any juices.
While the meat rests, sauté the mushrooms.
Heat 1 tbsp butter and the remaining oil in
a large frying pan, then tip in the mushrooms
with plenty of seasoning. Fry until golden
and just softened, then tip into a bowl.
Add the stock to the mushroom pan, then
reduce by two-thirds. Tip in the mushrooms
and balsamic, then add the remaining butter
and let it melt. Tip in any meat juices, then
season to taste and keep warm. Thickly slice
the beef and spoon over the sauce. Serve
with mash and spinach.