Limoges-style potato pie

Method

1. Unroll both sheets of pastry and cut out a 25cm disc from one piece and a 28cm disc from the other, rolling the pastry out a little more if necessary. Place the smaller disc of pastry on a lightly greased non-stick baking sheet. Mix the creams together and set aside.
2. Bring a large pan of well salted water (1 tsp per 600ml) to the boil. Thinly slice the potatoes, ideally on a mandoline. Drop them into the boiling water and simmer for 3 minutes, until just tender when pierced with a small sharp knife. Drain and refresh under running cold water. Drain once more, spread the slices out on a clean tea towel, then pat dry thoroughly.
3. Tip the potatoes into a large mixing bowl and gently stir in half the cream mixture, the chopped shallot, garlic, nutmeg, ¾ tsp salt and a little pepper. Arrange the potato slices in an overlapping pattern in the centre of the smaller circle of pastry, leaving a 2.5cm border clear around the edge.
4. Beat one of the egg yolks with 1 tsp cold water. Brush a little of the mixture around the edge of the pastry, lay the second larger circle of pastry over the top of the filling and press the edges together well to seal, pressing out any trapped air as you do so. Crimp the edges between your thumb and forefinger to give an attractive finish, then chill the pie for 20 minutes. Meanwhile, preheat the oven to 220°C/fan 200°C/gas7.
5. Brush the top of the pie with the remaining glaze, then, with the tip of a sharp knife, score the pastry lightly into portion-sized sections and score a 2.5cm-wide circle in the centre of the lid. Bake in the centre of the oven for 20-25 minutes until puffed up and richly golden, then lower the temperature to 180°C/fan160°C/gas 4. Cover the pie with a loose sheet of foil if it seems brown enough, then continue to bake for 10 minutes or until the potatoes feel tender when you pierce the centre of the pie with a skewer.
6. Gently warm the remaining cream in a small pan, remove from the heat and mix in the remaining egg yolks, the mixed herbs and ¼ tsp of salt. Take the pie out of the oven and turn the temperature back up to 220°C/fan200°C/gas 7. Cut out the scored circle from the centre of the pie and gently make a small gap between the potatoes and the underside of the pastry lid, using the handle of a teaspoon if necessary. Very slowly pour in the egg and herb cream, tilting the tray backwards and forwards to encourage the cream to filter into the filling. Do this slowly and carefully, as it might not all fit in. Replace the pastry circle.
7. Return the pie to the oven, cover with foil if you think it’s browning too quickly, then bake for a further 10 minutes. Remove from the oven, leave to cool for 5 minutes, then slice into wedges to serve.