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Soak the dried porcini in 600ml kettle-hot water and set aside to cool, then drain, squeezing out the mushrooms and reserving the liquid. Heat half the butter in a pan and fry the onions for 7-8 mins until golden, then add the garlic and thyme and cook for a few minutes more. Add the remaining butter to the pan, then the soaked mushrooms and chestnut mushrooms. Turn up the heat and sizzle everything for 5 mins until the mushrooms are cooked and golden. Pour in the port and simmer to a sticky glaze, then the mushroom soaking liquid, and crumble in the stock cube before boiling everything rapidly for about 5 mins. Drain the mushroom mix using a sieve and leave to cool. Reserve and chill the liquid to use later for gravy.
When the mushrooms are cool, butterfly the turkey breast by cutting into one side of it so you can open it like a book. Cover the meat and use a meat mallet or rolling pin to bash it out to a rectangle about 3cm thick and a little smaller than A4 paper. Trim the edges and top to neaten it up, reserving the trimmings. Tip the turkey trimmings, mushrooms, parsley, lemon zest and parmesan into a food processor and season generously. Pulse to make a rough stuffing. Form the stuffing into a long sausage positioned along the long edge of the turkey breast. Roll the turkey to encase the stuffing and set aside.
On a lightly floured surface, roll out the pastry to a rectangle about 10cm wider and longer than the turkey was when you beat it out. Trim the edges, reserving the trimmings. Lightly brush the pastry all over with the egg yolk. Lay the turkey in the middle of the pastry. Fold the shorter edges over the turkey, then roll up the whole thing to encase. If you like, re-roll the trimmings, cut into Christmas shapes and use to decorate. Brush all over with egg yolk, place on a tray, seam-side down. Chill for at least 30 mins or up to two days.
To bake, heat the oven to 190C/170C fan/gas 6 with a tray inside. Brush the wellington with more egg yolk and season with sea salt flakes. Transfer to the hot tray and bake for 1 hr 15 mins until the pastry is golden and crisp. While the wellington is cooking, simmer the reserved mushroom liquid until reduced to a sticky gravy. Leave the wellington to rest, then serve in thick slices with the gravy.