Begin by finely chopping the mushrooms in a food processor with a pinch of seasoning until you achieve a rough, coarse paste. Transfer this paste into a hot pan and cook vigorously over high heat for about 10 minutes, tossing frequently to evaporate excess moisture and concentrate flavors. Spread the cooked mushroom mixture on a plate to cool – this step is crucial to prevent sogginess later. Warm a dash of olive oil in a frying pan until shimmering. Season the beef fillet generously, then sear it quickly for about 30 seconds on each side. The goal here is to develop a beautiful caramelized crust that locks in juices, not to cook through. Remove the beef from the pan and let it cool slightly before brushing all over with English mustard, adding a subtle sharpness. Lay a sheet of cling film on your work surface and neatly arrange the Parma ham slices in slightly overlapping rows to form a base. Using a palette knife, spread the cooled mushroom paste evenly over the ham. Position the seared beef fillet in the center. Carefully wrap the ham and mushroom mixture around the beef using the cling film to help form a tight, compact barrel shape. Twist the ends securely and chill in the fridge for 15 to 20 minutes to firm up the shape for easier handling. On a floured surface, roll out the puff pastry to a rectangle about the thickness of a £1 coin. Remove the cling film from the beef parcel and center it on the pastry. Brush the edges generously with beaten egg yolk to help seal the pastry. Fold the shorter ends over first, then wrap the sides around the beef, trimming any excess dough. Turn the wrapped fillet seam-side down onto a baking tray. Brush the entire pastry surface with more egg yolk and chill again for about 15 minutes to let the dough rest and hold its shape. Preheat your oven to 200°C (400°F, gas mark 6). Lightly score the pastry at 1 cm intervals to allow steam to escape, then glaze the surface once more with egg yolk for a golden finish. Bake for 20 minutes, then reduce the oven temperature to 180°C (350°F, gas mark 4) and continue cooking for another 15 minutes. This staged baking ensures a crisp, evenly browned crust. Once baked, let the Beef Wellington rest for 10 to 15 minutes before slicing. Resting is key to retaining juicy, tender beef with a perfectly pink center. Serve slices accompanied by your favorite sides such as roasted potatoes, green vegetables, and a rich red wine sauce.