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Heat the oil in a frying pan over a high heat, then season the pork loin and sear until well browned all over – use tongs to sear the ends. Set aside on a board and leave to cool. Reduce the heat, add the butter to the pan and cook the onion for 10 mins until golden. Leave to cool.
Meanwhile, tip the chicken, mushroom powder, a little salt and the truffle oil, if using, into a food processor. Blitz to a coarse paste. Scrape down the sides and pulse in the whole beaten egg until fully incorporated. Drizzle in the cream, then add the chestnuts, fried onions, sage and a grinding of black pepper. Pulse again to bring everything together – the mixture should resemble a coarse mousse or pâté.
Lay a large sheet of baking parchment on your work surface, and arrange the pancetta rashers on top in two rows, slightly overlapping each other so they create a rectangle that’s large enough to wrap around the pork. Spoon the chicken mousse into the middle of the rectangle, and spread with a palette knife or spatula so it covers the pancetta. Lay the cooled pork in the middle. Use the parchment to help you draw the mousse-coated pancetta around the pork, then twist the ends like a Christmas cracker. Transfer to the fridge and chill for 1 hr.
Roll the pastry out on a lightly floured surface to a 35 x 30cm rectangle. Trim the edges to neaten (you can reserve the trimmings to make decorations later, if you like), then lightly brush all over with some of the egg yolk glaze. Carefully unwrap the pork from the baking parchment, and lay in the middle of the pastry rectangle. Fold the shorter edges of the pastry over the pork, then roll the whole thing up to encase the pork tightly. Line a baking tray with baking parchment. Put the wellington, seam-side down, on the tray. Chill for at least 30 mins. Will keep chilled for up to 24 hours.
Heat the oven to 200C/180C fan/ gas 6 with a baking tray inside. Re-roll the pastry trimmings and cut out pastry leaves, if you like. Unwrap the wellington and brush all over with more of the glaze, score the pastry lightly with a sharp knife, and decorate with the leaves, if using. Brush the leaves with the remaining glaze, then sprinkle with sea salt flakes. Carefully transfer to the hot tray in the oven, along with the baking parchment, and bake for 1 hr until the pastry is golden and the pork is just cooked through. If the pastry starts to get too dark, cover with foil. The pork is cooked when a cooking thermometer reads 65C. Leave to rest for at least 10 mins, then serve in thick slices with mushroom sauce and roast potatoes (see 'goes well with' below).