Preheat oven to 160C. Heat oil in a flameproof casserole. Add beef fillet and brown on all sides over high heat for 7 minutes, then remove and cool beef on a wire rack. Season to taste.
Heat butter in a frying pan and sauté mushrooms and shallot over medium heat, until all the water has been released.
Put the mushroom and shallot mixture with the parsley and bread into a food processor, and process to a fine stuffing mixture. Season to taste.
Spread mushroom stuffing along the length of the fillet.
Roll out the puff pastry to a length that will completely wrap around the beef fillet. Brush the edges of the pastry with egg yolk, then lay the beef fillet on the pastry and wrap it up tightly like a parcel, pressing the seams together firmly and tucking the ends under. Put the parcel seam-side down into the casserole, then brush all over with remaining egg yolk.
Make a couple of vents in the pastry to allow steam to escape, then bake for 35 minutes until golden and cooked rare. Rest for 10 minutes before serving.
Note Fresh cèpe mushrooms are difficult to find in Australia;
substitute king brown mushrooms.