Preheat the oven to 180C/350F/Gas 4.
Season the beef with the thyme, sea salt and pepper and rest for 30 minutes.
Heat a pan and sear each side of the beef until golden brown.
Allow to cool to room temperature.
For the Duxelles, sweat the butter, garlic and mushrooms in a pan over a low heat until all the moisture evaporates.
Add the truffle oil and parsley, and season to taste.
Remove from the heat and allow to cool to room temperature.
Smear the beef with an even coating of English mustard.
Lightly dust a sheet of baking paper with flour.
Roll the pastry so it is a little wider than the beef, and the beef can be completely rolled in pastry.
Place the pastry so the longest half is facing you.
Spread the Duxelle mixture evenly over the half of the pastry closest to you.
Place the beef on top of the mushroom mix.
Roll the beef up in the pastry, leaving a slight overlap of 3cm/1¼in. Brush this with beaten egg and seal.
Trim the ends of the pastry so they are flush with the beef.
Cut a sheet of baking paper to the size of the beef Wellington.
Place the beef Wellington on the paper, lightly brush the top with melted butter and refrigerate for 30 minutes.
Put the beef Wellington in the oven and cook for 25 minutes or until dark golden in colour.
Serve with steamed spinach or roast vegetables of your choice.