Make a pepper, garlic and butter rub and massage the fillet all over. Stand 1 hour on the bench.
Pan fry for 5 minutes, roll constantly and do both ends. Rest, then wrap tightly in clingwrap. Refrigerate.

Chop the mushrooms, garlic, herbs, and butter together. Cook in a large frypan over low heat for 30 minutes or more to remove most of the moisture. Mash with a wooden spoon while adding the truffle oil. Allow to cool.

Layout a sheet of clingwrap 400mm long. Lay out strips of prosciutto ovelapping and wider than the beef. Make sure its length is greater than the circumference of the fillet. Cover with the mushroom mix and press flat. Remove the fillet from its wrapping and set in the middle and roll up tightly around the fillet. Take care to make neat. Massage to remove air pockets and make a tight forming skin. Wrap tightly in extra clingwrap. Refrigerate

Scald the milk, water and butter and cool to <50°. Sift the flour and salt into a mixer bowl. Add the eggs and half the warm milk. Beat 2 minutes on speed 3 with the K attachment.  Add the parsley and thyme and the remaining milk and whisk 3 minutes on speed 4. Stand in a jug for 30 minutes.

Heat a new large non stick fry pan to low. Slightly cool the pan by waving like a fan before adding batter. If using an older pan spray with a little canola oil to prevent sticking. Stir the batter and then pour 80ml into the pan in such a way as to make a rectangle shaped crepe. Tilt to form the shape and thin the crepe. Cook 45 seconds, flip and cook 45 seconds. Set aside to cool. Continue till all the batter is used.

Layout more clingwrap and layer with the crepes overlapping. Unwrap the cold fillet and set onto the crepes. Fold in the ends first and trim to fit. Roll up the crepes using the clingwrap. Massage to remove air pockets and make a tight forming skin.Tightly wrap in more clingwrap. Refrigerate

Join 2 sheets of thawed puff pastry. Use melted butter at all the joins. Cut 25mm longer than the circumference of the fillet. Make 2 end caps with the remainder. You need a 20mm edge bigger than the size of the end. Unwrap the beef and set onto the pastry. Set the end caps in place. Lift to set under by 20mm and then form around the end. Roll up tightly and join. Wrap tightly in more cling wrap. Massage to connect all the joins positively. Its important to make sure all the joins in the pastry are neat and secure. Wrap very tighltly in more clingwrap. Refrigerate 24 hours or more.

Unwrap and set on a baking tray (and brush with egg wash, optional). Stand for 1 hour. Score concentric rings. Cut a vent hole and insert a piping tip to act as a chimney. Bake on a tray at 230° for 15 minutes then 180° for another 35 minutes. Rotate to promote even cooking.

Target temp is 61° after resting. A simple creamed spinach is perfect. It should be so moist and juicy that no sauce is required. However a Bearnaise is probably my choice for another level of sophistication.

This is a very technical dish and even if its not a complete visual success first time it will still be delicious. Issues that can be avoided. Use the best fillet you can afford. . Use puff pastry bought recently. Score only concentric rings as carving guides. Best not to cross hatch the pastry. Make the mushroom mix fairly dry. Use a new or near new non stick pan to make crepes.  Ensure a resting time of an hour.

900g  Eye Fillet Trimmed
Pepper Garlic Butter rub

Mushroom Mix::
400g Mushrooms (various)
2 Cloves Garlic
20g Butter (soft)
1 tsp each Dried Tarragon and Chives
1/2 Tbs Chopped Parsley
2 tsp Truffle oil (optional)

6 large Prosciutto Slices cut thick

Crepes::
150g Plain Flour
1/4 tsp Salt
2 Eggs
200ml Milk
200ml Water
50g Butter
1 Tbs Parsley and Chives

2 sheets Puff Pastry (new packet)
Egg wash (optional)

The two end bits without meat will be one of the tastiest pastries you will ever eat. Worth cooking just for these fantastic morsels.

A cheaper fillet almost certainly has been frozen at some stage. This will give up a lots of moisture that needs to be drained away in the cooking process. Deal with a quality butcher to get a fresh properly aged piece of prime meat. Its worth it!

Otherwise to drain while roasting cut a small hole at the bottom on one end. Tilt the oven rack by setting one side a row higher. Obviously the hole must go to the low side.
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I have cooked quite a few Beef Wellingtons. Mostly well done individual versions. Modern tastes want a rare fillet with a crispy puff pastry. Good trick with a 100 year old recipe meant to be well done. You need a large fillet from the butt end that weighs 1.3kg or more before trimming.

I do 1 step per day and its perfect. But it can be knocked up in just a few hours or a day if need be. Once it is tightly wrapped in the final layer of puff pastry it will keep fine refrigerated for a week. Refrigerate in this recipe can mean an hour or a day. 3 hours is OK. 1 hour and the end product can be loose.

Getting rare to medium rare is the target. Chill the assembled roast to its core. Then, let the outer pastry get to a warmer temperature sat on the bench for say 30 minutes whilst the insulated core stays really cold. Only then is it possible to keep the fillet rare with a well cooked pastry.