Wrap the beef fillet in cling film to form a barrel shape and put in the fridge for at least 1 hour.
Season the beef with salt, pepper and thyme.
Heat some oil in a saucepan and seal the beef, allow to cool once fully sealed.
Roughly chop mushrooms and then place in a blender until it is a puree texture.
Heat in a saucepan to remove all moisture.
Lay the pancake , overlapping slightly.
Spread the mushroom over the pancake.
Cover the beef with mustard.
Place the beef in the centre and then wrap the pancake to fully cover the beef. Cover with cling film to form a barrel shape and place in the fridge for 30 minutes.
Roll out the pastry into a square shape so that it is big enough to completely cover the beef.
Lightly hand whisk the egg to create an egg wash and cover the pastry.
Place the beef in the centre of the pastry and roll it tight until the beef is fully covered.
Add some egg wash to the edges and fold these over.
Wrap again in cling film and place in the fridge for 15 minutes.
Pre-heat oven at 200 degrees.
Remove wellington from the fridge and cover in egg wash.
With a fork lightly create a pattern in the egg wash.
Place in the oven and cook for 40 minutes.
Allow to rest for 15 minutes.
Remove the edges and then serve.
