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The night before, prepare the peppercorns for the sauce. Put the green peppercorns and vinegar in a small bowl, cover and leave at room temperature overnight.
The next day, pat the beef dry with kitchen paper, cover loosely, and leave for 1 hr at room temperature – this helps to ensure perfectly cooked meat later. Put the dried porcini mushrooms and black peppercorns in a small blender or spice grinder, and blitz to a powder. Mash with the butter and set aside.
Heat the oven to 230C/210C fan/gas 8. Season the sirloin all over with the sea salt. Heat the oil in a flameproof roasting tin or large frying pan over a high heat, and sear the beef, fat-side down, for 5-8 mins until some of the fat has rendered out and turned golden brown, then holding the beef with tongs, sear it on all sides for 2-3 mins until browned all over. Sit the browned beef in the roasting tin fat-side up and spoon out most of the fat (if you have rendered it in the tin). Spoon 2 tbsp of the rendered fat over the beef and reserve any leftover fat for making yorkshires. Arrange the shallots around the beef, turning to coat them in the fat.
Roast for 20 mins, then dot the porcini butter all over the beef and reduce the heat to 180C/160C fan/gas 4. Roast for another 40-50 mins for rare to medium-rare, or 1 hr for medium-well. If you have a meat thermometer, it should read 55C for medium-rare, or 65C for well done. Baste the meat in the butter from the tin, then transfer to a warm platter or board with the shallots. Cover and leave to rest for 20 mins.
To make the sauce, discard all but 4 tbsp fat from the tin and set the tin over a low heat (or transfer to a pan), and fry the chopped shallots for 6-8 mins until softened and translucent. Strain the pickled peppercorns, reserving the vinegar, and add to the pan along with the stock. Bring to a simmer and cook for 10 mins until reduced by about half, then whisk in the cream. Taste for seasoning, and add the reserved vinegar, 1 tsp at a time, to balance the acidity and creaminess (we used about 5 tsp). Simmer for another 5-6 mins until the sauce has thickened. Pour into a jug. Carve the beef and serve with the roasted shallots and sauce.