Photo: Nassima Rothacker
Preheat the oven to 230°C, fan 210°C, gas 8. Pat the fillet dry with kitchen paper. Remove any fine sinew or fat from the beef. Season with salt and pepper. Pour the oil into a large flameproof roasting tray large enough to hold the fillet. Set over a high heat on the hob. Once hot, add the meat and brown on all sides – this should take about 5 minutes. Then roast in the oven for 20 minutes for rare meat, 30 minutes for medium, or 45 minutes for well-done beef.
Remove the fillet from the oven (leave the oven on to bake the pastry) and transfer to a rack over a cold roasting tray and leave to cool completely.
Meanwhile, melt the butter in a large frying pan over a medium low heat. Add the shallots and garlic and fry for 5 minutes or until soft and golden. Discard the shiitake stalks, finely chop their caps and add to the softened shallots. Fry briskly for 5 minutes or until they have collapsed and any liquid has evaporated. Season to taste and, once cool, mix in the parsley.
Take 250g of the pastry and roll it into a rectangle that is a little larger than the length and width of the beef fillet. Place on a lightly oiled large baking sheet. Prick all over with a fork and bake in the oven for 15 minutes or until lightly golden (you can turn the oven off now). Transfer to a clean baking sheet and leave to cool completely.
Once the beef and pastry are cold, spoon the cooked mushrooms along the length of the pastry. Top with the beef.
On a floured work surface, roll out 500g pastry into a large rectangle, large enough to easily cover the beef fillet and be tucked in under the cooked pastry. Gently lift the pastry and let it loosely cover the beef. Cut off the corners and any excess pastry, and save the trimmings.
Brush the underside of one length of the pastry with beaten egg and, using a palette knife, tuck it neatly under the cooked pastry. Repeat the process with the remaining three sides. Liberally brush the uncooked pastry with the remaining beaten egg. Cut the trimmings into stars and decorate the parcel.
Brush the pastry again with beaten egg and chill for 30 minutes to firm up.
Preheat the oven to 230°C, fan 210°C, gas 8. Bake the beef en croûte for 20-25 minutes or until the pastry is golden and crisp. Rest the meat for 20-30 minutes before serving.