Begin by trimming the beef tenderloin, removing any silver skin and excess fat. Cut the center portion, known as chateaubriand, for the Wellington.

Season the beef with avocado oil, Maldon salt, and black pepper. Sear it in a hot cast-iron pan on all sides until a nice brown crust forms.

Once seared, brush the hot beef with English mustard and allow it to cool in the refrigerator.

For the mushroom duxelles, finely chop the mushrooms, shallots, and garlic using a food processor.

Sauté the shallots and garlic in butter, then add the mushroom mixture. Cook over medium heat, stirring until most of the moisture has evaporated.

Add fresh thyme, deglaze the mixture with a splash of Lagavulin whisky, and continue to reduce until it becomes thick and paste-like. Season with flaky salt and black pepper, then chill.

Lay overlapping strips of prosciutto on cling film, spread a thin layer of the cooled mushroom duxelles over the prosciutto, and place the chilled beef in the center.

Roll the prosciutto and duxelles tightly around the beef, using the cling film to shape it into a uniform log. Refrigerate for at least 15 minutes.

On a floured surface, roll out the puff pastry and place the chilled beef log in the center. Wrap the pastry around the beef, sealing the edges.

Prepare an egg wash by mixing 1 egg yolk with 1 tablespoon of water, then brush the pastry all over. Chill again to firm up the pastry.

Roll out a second sheet of puff pastry, cut it into strips, and arrange them in a lattice pattern over the top of the Wellington. Brush the lattice with the egg wash.

Preheat the oven to 400°F (200°C). Insert a temperature probe into the center of the Wellington.

Bake for about 30 minutes, or until the internal temperature reaches 128–130°F for medium-rare.

While the Wellington is resting, prepare the pan sauce by simmering the chicken broth and reducing it by half. Whisk in the butter cubes and finish with a splash of balsamic vinegar.

Slice the rested Wellington into 1–1¼ inch thick slices, plate them, drizzle with the pan sauce, and garnish with flaky salt and fresh thyme.