To begin, season the beef fillet generously with salt and pepper. Heat olive oil in a heavy pan until very hot. Sear the beef on all sides, about 2 minutes each, until browned but not cooked through. Remove from heat, brush all over with mustard, and allow to cool completely. Move to preparing the duxelles.

In the same pan, melt butter and add shallots and garlic. Cook until softened. Add mushrooms and thyme, cooking over medium heat until the mixture is dry and concentrated, about 10 minutes. Season to taste, then cool completely. Transition to assembly.

Lay out cling film on a work surface. Arrange the prosciutto slices slightly overlapping to form a rectangle. Spread the cooled mushroom duxelles evenly over the prosciutto. Place the cooled beef fillet on top. Move to rolling.

Using the cling film, roll the prosciutto and duxelles around the beef to form a tight log. Twist the ends of the cling film to secure. Chill in the fridge for at least 30 minutes to firm up. Proceed to pastry preparation.

On a lightly floured surface, roll out the puff pastry into a rectangle large enough to encase the beef log. Ensure even thickness to prevent uneven baking. Transition to wrapping.

Remove the beef log from the cling film. Place it in the centre of the pastry. Fold the pastry over the beef, trimming excess and sealing the edges tightly. Brush with egg wash to seal. Chill for 15 minutes to set the shape. Move to oven preparation.

Preheat the oven to 200°C (fan 180°C). Place the wrapped Wellington on a lined baking tray, seam side down. Brush generously with egg wash. Use pastry trimmings to decorate if desired, brushing with more egg wash. Transition to baking.

Bake for 35–40 minutes until the pastry is golden and crisp, and the beef reaches 50–52°C internally for medium rare. Rest for at least 10 minutes before slicing to allow juices to settle. Move to plating.

Slice the Wellington thickly and arrange on a platter. Serve with roasted potatoes, green beans, or a rich red wine jus. For presentation, garnish with thyme sprigs or seasonal vegetables to highlight its festive character.