Trim the beef of any excess sinews and fat. Rub the oil over the beef and season.
Preheat a non-stick frying pan and sear the beef for about 2-3 minutes on all sides, until golden.
Remove the seared joint from the pan and stand on a wire rack set over a plate, whilst you prepare the rest of the recipe.
Melt the butter in the frying pan and fry the mushrooms for 5 minutes over a high heat until golden. Stir in the garlic and thyme, reduce the heat and cook for a further 1 minute. Remove the mushrooms from the pan and cool for 10 minutes. Mix the pâté and mushrooms together in a bowl, to make a thick paste.
Preheat the oven to 220ºC, fan 200ºC, gas 7. Open out the pastry sheet and spread the mushroom mixture over the middle two thirds of the pastry. Lift the beef on top.
Trim a little pastry from the sheet for decorations. Brush one seam of the pastry with egg and seal the pastry edges together. Brush the pastry ends with egg and seal the edges. Turn the joint over, and make sure the pastry is smoothed over the mushroom mixture. Lift on to a baking sheet.
Cut leaves from any excess pastry. Brush the wellington with more egg and decorate with the pastry leaves. Brush the leaves with egg and then score the pastry to form veins.
Bake in the middle of the oven for 30 minutes. Reduce the oven temperature to 200ºC, fan 180ºC, gas 6 and bake for a further 10 minutes. Remove from the oven and allow to rest for 10 minutes before carving. Serve with steamed broccoli.