To make the beef Wellington, trim the beef fillet, reserving the trimmings, and season well.
Heat a frying pan over a medium heat and sear the beef on all sides. Set aside to cool completely.
Once cold, cover in the paté and then the mushroom mixture. Wrap in the Parma ham.
Roll out the pastry on a lightly floured surface and cut in half. Sit the beef in the centre of the pastry, then cover with the remaining pastry and seal with a little egg wash. Brush the Wellington with egg wash and chill in the fridge for an hour.
Meanwhile, to make the jus, place a frying pan over a high heat, add the beef trimmings and colour on all sides. Add the shallots, celery, peppercorns, thyme and garlic and sauté until soft. Add the vinegar and the wine and bubble until the volume of the liquid has reduced by half. Add the stock and simmer for 1 hour, until reduced by two thirds.
To make the celeriac purée, boil the celeriac in the milk with the bay leaf until soft.
Add the cream and bring back to the boil. Strain into a blender and blitz until smooth. Add a knob of butter and blitz until you have a shiny purée. Keep warm.
Meanwhile, preheat the oven to 220C/200C Fan/Gas 7. Bake the Wellington for 20 minutes. Remove from the oven and cool.
Strain the reduced jus into a clean pan, season and add a knob of butter to emulsify.
Slice the Wellington and place on plates with some celeriac purée, and a drizzle of jus.