Preheat the oven to 220°C, fan 200°C, gas 7. Heat 1 tablespoon 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 1 tablespoon 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 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.