Season the beef well. Melt half the butter in a large frying pan over a medium heat. When foaming, brown the fillet all over for 4-5 minutes, taking care not to burn the butter. Put the beef on a plate and leave to cool.
Melt the remaining butter, add the shallots and cook for 1 minute. Drain the porcini, reserving the liquid. Chop the porcini, then add top the pan with the thyme, garlic, reserved liquid and flat mushrooms. Increase the heat and cook until the mushrooms are dry in the pan (this may take some time). Season and cool.
Put the pate in a bowl and mash to a paste, add the mushroom mixture and combine. Season. Transfer the cold fillet to a board, take off the string, pat dry with kitchen paper and use a palette knife to spread half the mushroom and pate mixture over one side of the beef.
Lay half the parma ham on cling film, with slices overlapping. Put the beef on to the ham, mushroom side down. Spread the remaining mushroom mixture over the beef, then overlap the rest of the parma ham on top, wrapping it round the sides to make sure it is all covered. Wrap in cling film and chill for about 20 minutes. Heat the oven to 220C/fan 200C/gas mark 7.
On a lightly floured surface roll out the pastry (sit 1 block on top of the other) into a piece big enough to wrap the beef. Prick lightly all over. Remove the cling film from the beef, put it in the middle of the pastry and brush all around it with egg. Fold the pastry over the beef to make a parcel with the join underneath. Press down the two edges, trim off any wodgy bits and tuck under the ends. Brush with more beaten egg and decorate with pastry off-cuts if you like. Brush again with egg and cook on a baking sheet for 15 minutes, then turn down to 200C/fan 180C/gas 6 and cook for a further 30 minutes for rare to medium rare or 45 minutes for medium. Leave to stand for 10 minutes before cutting.