Melt a little butter in a pan and sauté the onions and garlic until soft but with no colour. Add the mushrooms and thyme. Season with salt and pepper and cook for around 10 minutes or until cooked and there is no moisture. Stir through fresh parsley.
To make the crepes, Sift the flour into a bowl. Beat together the eggs and milk. Mix flour and egg mixtures together and add the melted butter, chopped chives and pinch of salt. Set aside to thicken for about 15 to 30 minutes.
Over a medium heat, melt a knob of butter in a frying pan. Ladle a little crepe mix into the pan and quickly swish around to cover the base. Cook briefly on both sides but without getting any colour on the crepe. Set aside. Keep going until you have at least 6 crepes.
Season the beef fillet with salt and pepper. Seal in a roasting pan or on a flat top grill until all sides are browned.
To assemble the wellington, lay out a piece of puff pastry long and wide enough to enclose the beef fillet. Cover with crepes (this soaks up the beef juice and stops the pastry from going soggy on the bottom).
Place a layer of mushroom mix in the centre, then place the beef fillet on top.
Roll over the crepes and pastry and neatly seal with eggwash. Place on a baking sheet. Decorate the pastry with shapes if desired.
Eggwash the top and refrigerate for 20 minutes to recool the pastry then bake at 220C for 45 minutes. If you have a thermometer, cook to about 45C in the centre of the beef, you should have a nice medium rare beef fillet. Serve with a nice rich beef glaze or red wine glaze.