Trim fillet neatly, removing ends and if desired, brush with brandy.
In butter, lightly saute mushrooms and onions until soft and transparent.
Allow to cool.
(This may be done ahead of time.) 1 hour before cooking, roll out puff pastry into a thin sheet, to a size which exceeds the dimensions of the fillet.
Spread parsley along the centre of the pastry in a 3 inch strip.
Add chopped egg on top of parsley cover with onion and mushroom mixture.
Sprinkle liberally with salt and freshly ground pepper to taste.
Place fillet on top and wrap pastry neatly around ii securing it at the ends.
Turn over and place on baking sheet, seam side down.
Make twig and leaf designs on top with remaining pastry.
Brush pastry with cold water and bake in hot oven 450 degree F for 12 to 15 minutes, then reduce oven to 375 degree F.
Brush pastry with slightly beaten egg yolk and continue baking until crust is browned, usually another 20 minutes.
Serve with Bearnaise Sauce