- 3 large potatoes, thinly sliced
- 2 tbsp olive oil
- 4 large leeks, cleaned, halved lengthways and chopped into large pieces
- 2 garlic cloves, finely chopped
- 350ml crème fraîche
- 2 large free-range eggs, lightly beaten
- 1 heaped tbsp wholegrain mustard
- 160g Gruyère, grated
- Bunch of fresh chives, chopped
For the pastry
- 200g plain flour, plus extra for dusting
- 200g wholemeal flour
- 1 heaped tsp mustard powder
- 150g butter, chopped
- Milk for glazing
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.