1. You will need a 25cm (10in) round pie dish.
2. Leave the butter and lard for the pastry in the freezer for 1 hour.
3. To make the pie filling, melt the butter in a wide-based saucepan or deep frying pan, add the leeks and onion and fry over a high heat for 2–3 minutes. Cover with a lid, then lower the heat and cook for 10–15 minutes or until
tender but not browned.
4. Meanwhile, boil the potatoes in salted water in a separate pan for 8–10 minutes until just tender. Drain well. Heat the milk in a separate pan until just coming to the boil and remove from the heat.
5. Remove the lid from the pan with the leeks and onion, turn up the heat to drive off any excess liquid, then sprinkle in the flour. Stir in and cook for 1 minute, then gradually blend in the hot milk and bring to the boil, stirring.
Leave to bubble for about 5 minutes until thickened, then add the mustard, thyme and the two cheeses and season to taste with salt and pepper.
6. Add the cooked potatoes to the sauce and stir in. Pour the filling into the pie dish and sit a pie funnel (or an upturned eggcup) in the middle of the dish to support the pastry. Cover and set aside until the filling is stone cold
7. To make the pastry, measure the flour into a large bowl. Take the butter and lard straight from the freezer and coarsely grate into the flour (see tip). Add 150ml (5fl oz) of water and stir together to combine into a firm dough.
8. Sprinkle a work surface with flour, add the dough and knead until smooth. Roll into a rectangle about 20 x 50cm (8 x 20in), then fold up the bottom third of the pastry to the middle and fold the top third down over it, creating
three layers. Turn the pastry through 90 degrees, roll again and fold, then turn and fold twice again to give 12 layers in total. Wrap in cling film and chill in the fridge for a minimum of 30 minutes.
9. Meanwhile, preheat the oven to 220°C/200°C fan/Gas 7.
10. Roll the pastry on a floured work surface until about as thick as a £1 coin and cut out a disc 3cm (1¼in) bigger than the pie dish. Cut a long thin strip about 1cm (½in) wide to fit around the top of the dish. Wet the lip of the
dish, lay the thin strip of pastry on top and press down (see tip). Wet the top of the pastry lip and lay the pastry disc on top, pressing down and crimping the edge.
11. Re-roll the pastry trimmings and cut out leaf shapes for decoration (see tip). Brush the top with some of the beaten egg and arrange the leaves on top. Make a little hole in the centre. Brush the top of the pie with more
beaten egg and sit it on a baking sheet.
12. Bake in the oven for 40–45 minutes or until crisp and golden on top and the filling is piping hot. Serve with a green vegetable.
Mary Berry's cooking tips
The base can be made and left in the pie dish up to a day ahead. Top with the pastry up to 6 hours before cooking. The pastry can be prepared ahead and chilled in the fridge overnight.
Both the pastry and the pie freeze well uncooked.
Wrap the butter and lard blocks in kitchen paper and use this to hold the fat while grating.
The filling needs to cool down completely as any residual heat will start melting the pastry and cause it to sink when baking in the oven.
Adding a strip of pastry to the edge of the pie dish helps raise the pastry a little and prevents it from going soggy during cooking.
Don’t re-knead the trimmings – lay them on top of each other and then roll, as this keeps the layers in the pastry intact.
This recipe is extracted from Mary Berry Everyday, BBC Books, RRP £26.00.