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Preheat the oven to 200°C/180°C fan/Gas 6. Put the joint of beef into a roasting tray, brush all over with a little oil and season well with salt and pepper, then roast for 15–20 minutes, until it reaches an internal temperature of 36°C. Transfer the joint to a sealable container and set aside to cool. Once cool, cover the container with its lid, then chill in the fridge for at least 2 hours, until cold, or up to 12 hours, before serving.
For the dressing, melt the butter in a small frying pan, then add the mushrooms and sauté over a medium heat for 1 minute. Set aside to cool.
Once cold, finely chop the mushrooms and place in a bowl. Add the crème fraîche, olive oil, truffle oil and sherry vinegar and whisk together until combined. Season to taste with salt and pepper.
Smear the dressing onto four serving plates. Slice the chilled beef as thinly as possible using a very sharp knife, then lay the slices on top of the dressing – depending on how thinly you slice the beef, four or five pieces per serving will be plenty (see Tip). Garnish with frisee, chives, beetroot and cress. Finish with shavings of black truffle, if using, and season with pepper.
TIP Store any leftover (unsliced) beef in an airtight container in the fridge for up to 3 days, then slice it thinly and use in sandwiches, salads or as part of a cold meat platter.