Heat the olive oil and ½ oz of the butter in a non-stick frying pan. Season the beef. When the pan is hot, add the beef and quickly sear all over.
Remove from the pan, place on a plate and allow to cool completely.
To make the stuffing, heat the rest of the butter in the frying pan until foaming. Add the shallots and cook until lightly golden, then add the mushrooms and cook gently, stirring until all the liquid evaporates.
Stir in the cream and season generously. Continue to cook until the mixture has reduced to a thick paste. Stir in the parsley and leave to cool.
Preheat the oven to 450ºF. Mix the egg yolk in a small bowl with the milk and set aside to use as an egg wash.
Unroll one of the packets of puff pastry on to a large baking sheet lined with parchment paper and lay two pancakes on top, slightly overlapping. Spread the chicken liver pâté in a strip across the centre of the pancakes, to the same width as the beef fillet.
Make a lengthways cut into the centre of the beef about three-quarters of the way through and fill with the cooled mushroom stuffing.
Place the beef on the pancakes and cover with the two remaining pancakes, slightly overlapping. Tuck the bottom pancakes up the sides of the beef and under the top pancakes.
Brush the egg wash around the pastry edges and lay the second unrolled packet of puff pastry on top, sticking the edges down to seal by pressing the tip of a teaspoon down gently.
Decorate the top with the pastry trimmings if liked, and brush all over with the egg wash. Place in the fridge for 20 minutes to allow the pastry to rest.
Place the beef Wellington in the oven for 10 minutes, then reduce the heat to 375°F and continue to cook for another 20 minutes until the pastry is puffed up and golden. Leave to rest on a carving board for 5 minutes.
To serve, carve the beef Wellington into thick slices and arrange on warmed serving plates with the Madeira sauce, baby carrots and baby new potatoes.