Heat the oil and butter in a non-stick frying pan, when hot add the seasoned beef and seal on sides. Remove from the pan and allow to cool completely.
To make the duxelle stuffing, in the same frying pan add the butter and when
hot and foaming, add the shallots and cook until softened and golden. Add the mushrooms and cook until all the liquid evaporates.
Add the cream, salt and pepper. Continue to gently heat until it has reduced to a thick purée. Set aside to cool completely.
preheat the oven to 230C/450F/Gas8.
Roll the pastry to fit the beef and lay two of the pancakes on top, slightly overlapping. Spread a strip of chicken liver pate across the centre of the pancakes - to the same width as the beef fillet.
Make a cut about ¾ of the way through the beef into the centre of the beef fillet and fill with the cooled duxelle mix. Place the beef on to the pancakes and cover with the remaining two pancakes. Cut away the middle of the ends of the pastry. Fold over the main part of the meat and then neatly fold the ends.
Brush the edges with a little water. Place seam side down on a large baking tray. Decorate the top with extra pastry if desired and then brush all over with the beaten egg.
Place it in the oven for 10 minutes, then reduce the heat to 190C/375F/Gas 5 and continue to cook for another 20 minutes, until the pastry is golden.
Remove from the oven to a carving board and allow to rest for 5 minutes before carving into thick slices.