Prepare the Mushroom Duxelles:

Heat the vegetable oil in a frying pan and sauté the chopped mushrooms with seasoning for about 5 minutes, until golden brown. Add the shallot and garlic, and cook, stirring occasionally, for an additional 3-4 minutes. Remove from heat, set aside to cool, and finely chop once cooled.

Make the Chicken Mousse:

Blend the chicken fillet, egg whites, and salt in a food processor until smooth. Transfer to a bowl and fold in the prepared mushroom mixture along with the double cream. Chill the mousse in the fridge while you prepare the crêpes.

Prepare the Crêpes:

In a bowl, combine the flour with a pinch of salt. Make a well in the center and crack in the eggs. Gradually whisk the eggs into the flour, followed by the milk, to form a smooth batter. Lightly grease a non-stick frying pan (about 22 cm in diameter) with a little vegetable oil. Heat the pan over medium heat, then pour in just enough batter to cover the surface in a thin layer. Cook each crêpe for 1-2 minutes on each side until golden, then transfer to a plate. Repeat with the remaining batter, separating the crêpes with baking paper. Allow them to cool.

Assemble the Wellingtons:

Lay out four large pieces of clingfilm and place a crêpe in the center of each. Evenly distribute the chicken mousse across each crêpe, shaping it into a circle about the thickness of a pound coin, leaving a border around the edges. Season the beef fillets with salt and brush each fillet with Dijon mustard. Position a fillet at the center of each crêpe, then fold the crêpe over to encase the beef, trimming any excess and using spare crêpes to patch any gaps. Roll each bundle tightly in clingfilm and refrigerate for at least 30 minutes.

Prepare the Pastry:

On a lightly floured surface, roll each puff pastry sheet into a 25cm x 35cm rectangle. Cut each sheet in half to create four smaller rectangles. Unwrap the beef parcels from the clingfilm and place each on a pastry rectangle, leaving enough border to seal. Brush the edges of the pastry with the egg wash. Fold over the pastry, sealing the edges with a fork to create a neat pillow shape. Trim any excess pastry and transfer the Wellingtons to a baking tray lined with parchment paper. Brush them with egg wash again and, if desired, sprinkle with poppy or sesame seeds. Chill for at least 1 hour, or freeze for longer storage, adding extra baking time if cooking from frozen.

Cook the Wellingtons:

Preheat the oven to 220°C (fan 200°C, gas 7). Place a large baking tray in the oven to heat. Once hot, transfer the Wellingtons to the preheated tray and roast for 20-25 minutes for a medium-rare result. If using a digital probe, the internal temperature should reach 50°C for rare, 60°C for medium-rare, 65°C for medium, 70°C for medium-well, or 75°C for well-done. Allow the Wellingtons to rest for 20-30 minutes before slicing in half. Serve with gravy and your choice of seasonal vegetables.