Creamy leek, potato and Gruyère pie

Method

1. Make the pastry first. Put the plain flour, the wholemeal flour and the mustard powder in a large bowl with a good pinch of salt. Add the butter and rub in with your fingertips until the mixture resembles coarse breadcrumbs. Add ice-cold water 1 tbsp at a time and mix with a round-bladed knife until the mixture forms a dough. Tip out onto a lightly floured surface and knead gently until smooth. Wrap in cling film, then chill in the fridge while you make the filling.
2. Bring a medium saucepan of water to the boil, add the sliced potatoes and cook for 3-4 minutes until beginning to soften. Drain well and set aside.
3. Heat the oil in a large frying pan, add the leeks, cook for 2-3 minutes, then add the garlic and cook for a further minute. Remove the pan from the heat and leave the leeks and garlic to cool slightly.
4. Mix together the crème fraîche, eggs, mustard, most of the cheese (reserving about 3 tbsp), the chopped chives and a good pinch of salt in a large bowl. Stir in the cooled leeks and garlic.
5. Remove the pastry from the fridge. Cut away one-third of the pastry and set aside. Roll out the larger piece of pastry on a lightly floured surface and use to line the bottom and sides of a 23cm x 5.5cm deep loose-bottomed cake tin. Cover the base of the pie with the well drained, sliced potatoes, then pour the creamy leek mixture over the potatoes.
6. Roll out the remaining piece of pastry until it is 2.5cm larger than the top of the cake tin. Brush the edges of the pastry with a little milk, then press it on top of the pie to make a lid, crimping the pastry edges together to seal. Wrap the pie first in cling film, then foil. Freeze until needed.
7. The day before you want to eat the pie, remove it from the freezer and defrost completely overnight in the fridge. Preheat the oven to 180°C/fan 160°C/gas 4 and put a baking sheet in the oven to heat. Brush the top of the pastry with milk, put the pie on top of the heated baking sheet and bake for 35-40 minutes. Scatter the remaining cheese over the top and return to the oven for 5-10 minutes until the pastry is golden and crisp. The filling should still be oozy but hold its shape when cut into wedges.