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To make the sauce, heat the olive oil in a large frying pan. Add the shallots and mushrooms, then fry for about 10 mins until golden and caramelised. Pour in the vinegar, then simmer until almost evaporated. Add the red wine, then simmer until almost evaporated. Add the beef stock, bring to the boil, then reduce by two-thirds. Pass the mixture through a fine sieve, then keep warm until ready to serve. Can be made the day before and reheated.
Heat oven to 200C/fan 180C/gas 6. Season the beef generously with salt and pepper. Heat the sunflower oil in a large ovenproof frying pan on a high heat. Add the beef, then sear for 10 mins, turning, until browned all over. Transfer to the oven, then roast for approximately 20 mins, turning after 10 mins. This will give you beef that is medium rare; for medium, roast for an extra 5 mins. Allow the beef to rest on a rack, loosely covered in foil, for 10 mins, then slice into 6 thick slices. Add all juices from the beef to the sauce, then gently heat the sauce up and whisk in the butter.