Trim the fillet of any membrane, then season all over with salt and pepper. Heat the clarified butter in a roasting pan over a medium-high heat and sear the beef for 3-4 minutes until golden, turning to colour evenly. Transfer to the oven and roast for 8 minutes, turning the meat over after 4 minutes. Lift the beef onto a wire rack and leave to cool completely. AT this stage, it will be very rare.
For the mushroom duxelle, heat the 80g butter in a pan over a medium heat. Add the mushrooms and lemon juice and cook, stirring from time to time, until all the moisture has evaporated. Add the shallot and cook for another 2 minutes, then pour in the cream and cook, stirring, until it is all absorbed. Add the chopped parsley and season to taste and set aside to cool, then chill.
Blanch the spinach leaves in boiling salted water for 30 seconds, then drain and refresh in cold water. Drain well, separate the leaves and pat each one dry with kitchen paper.
To assemble, roll out the pastry on a lightly floured surface to about 40 x 25 cm rectangle, 3-4mm thick. Trim the sides to neaten. Place 2 crepes along the middle of the rectangle and cover them with 8 spinach leaves. Spoon and spread a 1cm thick band of mushrooms duxelle along the middle of the spinach crepes. Put the cold beef fillet on top and thickly cover the whole surface, including the ends, with the remaining duxelle.
Cover the mushroom duxelle with the remaining spinach leaves and crepes. Fold the crepes over the beef and if necessary cut off any overlapping parts with scissors. Lightly brush the 2 ends of the pastry rectangle with eggwash. Fold one side over the beef, brush it and the ends with eggwash, then fold to the side over the beef.
Roll out the 2 ends of the pastry to a 4-5mm thickness, and trim to an 8cm length. Brush these with eggwash and fold them over the beef. Turn the pastry-wrapped beef over onto a baking sheet, eggwash the entire surface of the pastry and refrigerate for 10 minutes. Meanwhile, heat the oven to 200 C / Gas 6. Brush the entire surface of the pastry with eggwash a second time and score it with leaf patterns using a knife tip. Cut a small aperture in the middle of the pastry to allow the steam to exscape during cooking. Bake for 25 minutes if you like your beef rare of 35 minutes for medium. If the pastry becomes too brown as it cooks, cover loosely with foil and lower the oven setting to 170 C / Gas 3.
Use a palette knife to transfer the cooked beef en croute to a wire rack and leave it to rest for 5 minutes. Carve 3 thick slices of the beef en croute and transfer the fillet onto a serving platter. Disregard the first slice with the crust, arrange the second slice on a plate and the third in front of the beef on the platter.
Garnish a la Biscontine (Escoffier)
Place 4 croustades made from Pommes Duchesse filled with creamed puree of cauliflower on the platter and one croustade on the plate. Place 4 braised Gem lettuce on the platter and one on the plate. Arrange watercress for presentation and serve the Bearnaise Sauce separately.
Put the flour in a mound on the work surface and make a well. Put in the butter and salt and work them together with the finger tips of one hand, gradually drawing the flour into the centre with the other hand.
When the cubes of butter have become small pieces and the dough is grainy, gradually add the iced water and mix until it is all incorporated, but don’t over work the dough. Roll it into a ball, wrap in cling film and refrigerate for 10 minutes.
Flour the work surface and roll out the pastry into a 40 x 20 cm rectangle. Fold it into three and give it a quarter-turn. Roll the block of pastry into a 40 x 20 cm rectangle as before, and fold it into three again. These are the first 2 turns. Wrap the block in cling film and refrigerate it for 10 minutes.
Give the chilled pastry another 2 turns, rolling and folding as before. This makes a total of 4 turns, and the pastry is now ready. Wrap it in cling film and refrigerate for 10 minutes before using.