Clean the mushrooms, but do not wash - better to wipe with a damp cloth or use a brush. Slice the button mushrooms and then tear the other ones into thumb-sized pieces.
Heat 1 tbsp of the vegetable oil in a frying pan and add the shallots and bacon until the shallots are softened and the bacon is cooked. Using a slotted spoon, remove the shallots and bacon but leave the oil in the pan.
Add the mushrooms and increase the heat to high. Leave the pan for 2 minutes until the mushrooms start to colour, turn the mushrooms and leave again until coloured. Repeat twice then return the shallots and bacon to the pan, mix well and add the madeira or port and cook until nearly all evaporated. Remove from the heat and leave to cool before stirring in the tarragon. Set aside.
Season the meat well with salt and pepper. Take a solid based fry pan and put it over a high heat for a good 5 minutes so it is seriously hot. Rub the meat with the remaining oil and sear the meat for 2 minutes on each side. Leave to cool. Once cool, spread the pate evenly over the surface of the beef.
Preheat the oven to 220°C/425°F/gas 7.
Roll out the pastry into a large rectangle about 2½ times as wide as the fillet and about 7-8 cms longer at both ends.
In the middle of the pastry, lay the three slices of prosciutto, then spread the mushrooms evenly on top, leaving a generous margin of pastry all around. Put the beef on top, and then brush around the edges of the pastry with a little of the egg, and fold the whole lot up, twist the ends like a bon bon and snap off any excess pastry.
Turn the parcel over onto a buttered tray so you have a rounded top pastry parcel brush with the egg but keep the dome shape, tucking the pastry at the ends underneath. Use the pastry trimmings to decorate, if you like, then brush any decorations with egg wash too.
Transfer to the oven and cook for 30-35 minutes, until the pastry is golden and well risen.
Serve immediately, cut into thick slices, garnished with some watercress.
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