Preheat the oven to 220°C/fan 200°C/gas mark 7. Heat 1 tbsp oil in a large pan over a high heat. Season the beef with freshly ground black pepper, then put in the pan and sear for 1 minute on each side, until browned all over. Set aside to cool.
Roll out a third of the pastry into a long rectangle, just slightly larger than the length and width of the beef fillet. Place on a non-stick baking tray and prick well with a fork. Chill in the fridge for 15 minutes, then bake in the oven for 12 minutes, until brown and crisp. Remove from the oven and allow to cool.
Heat the remaining oil in the pan. Add the mushrooms and shallots and cook over a medium heat for 5-10 minutes, until all the liquid has evaporated. Add the garlic and cook for a further minute. Tip into a bowl and stir through the pâté and herbs.
Roll out the remaining two-thirds of the pastry into a rectangle large enough to enclose the fillet. Place the beef on the cooked pastry and cover the top and sides with the mushroom mixture. Lay the rolled-out pastry over the top, tucking it under the pastry base to secure. Slice any trimmings into lengths and lay over the top to create a diamond effect, then brush the pastry with the egg yolk.
Cook in the oven for 30 minutes, then reduce the heat to 200°C/fan 180°C/gas mark 6 and cook for a further 15 minutes, until golden and crisp. Remove from the oven and rest for 10 minutes before slicing and serving.