Potato, mushroom and taleggio pie

Method

1. Roll out the savoury shortcrust pastry into a sheet roughly 4 mm (1/8 in) thick. Wrap the sheet in plastic wrap and place it in the fridge. Roll the puff pastry into a sheet roughly 3 mm (1/8 in) thick, wrap it in plastic wrap and place it in the fridge. Let the pastry rest after rolling it out – this will stop it shrinking once cut.
2. To make the filling, put the potatoes in a large saucepan, cover with water and season with salt. Bring to the boil, then reduce the heat and simmer for 20 minutes. The potatoes will fall off a sharp knife when done. Strain the potatoes and refresh them briefly under cold water. Quarter them lengthways, then cut into 2 cm (¾ in) slices and place in a large mixing bowl.
3. Strip the leaves from three sprigs of rosemary and finely chop. Heat 20 ml (¾ floz) of the oil and 20 g (¾ oz) of the butter in a large saucepan, then add the onion and garlic and sweat them off until soft and translucent. Remove from the heat, stir through the rosemary and add the mixture to the potato.
4. Heat a large frying pan over a high heat, then add the remaining oil and 15 g (½ oz) of the butter. Fry the mushrooms for 5 minutes, until softened and starting to colour, then add to the potato and onions.
5. Melt the remaining 40 g (1½ oz) of butter in a saucepan over a medium heat. Add the flour and stir over the heat until it forms a thick, sandy coloured paste. Gradually whisk in the milk, then continue to stir until the mixture is thick and comes to the boil. Remove from the heat, add the taleggio and continue whisking until smooth.
6. Pour the cheese sauce over the potatoes, onions and mushrooms. Add the tarragon and gently turn the mixture until everything is well coated in the sauce. Season to taste with salt and pepper. Set aside to cool completely.
7. Before you assemble the pies, preheat the oven to 180°C (360°F) and lightly grease your pie dishes. Lay the savoury shortcrust pastry on a lightly floured bench, and cut out 8 discs of pastry for the bases, 2 cm (¾ in) wider in diameter than the base of your pie dishes.
8. Line each pie dish with a round of shortcrust pastry and press it in well with your thumb. Trim off any excess and lightly brush the edges with the beaten egg. Lay out your puff pastry and cut 8 discs slightly larger than the top of your pie dishes. Fill each pie generously with the filling and place three slices of cornichon on top. Top each pie with a puff pastry lid and press the edges of the base and lid together to seal.
9. Brush the top of each pie with egg wash, then pierce it a couple of times with a sharp knife. Strip the leaves from the remaining sprig of rosemary and sprinkle them over the top. Bake at 180°C (360°F) for 10 minutes and then reduce the heat to 160°C (320°F), before baking for a further 30–40 minutes, until the pies are nice and golden. Allow them to rest for 10 minutes before eating.