Ingredients

• 125g/4½oz butter
• frozen for 1 hour
• 125g/4½oz hard vegetable fat
• such as Trex
• frozen for 1 hour
• 350g/12oz plain flour
• plus extra for dusting
• 1 free-range egg
• beaten for egg wash
• 60g/2¼oz butter
• 2 small leeks
• thinly sliced
• 1 onion
• thinly sliced
• 450g/1lb potatoes
• cut into 2cm/¾in cubes
• 600ml/20fl oz full-fat milk
• 50g/1¾oz plain flour
• 1 tbsp Dijon mustard
• 1 tbsp chopped fresh thyme
• 50g/1¾oz mature Cheddar
• finely grated
• 50g/1¾oz Parmesan or vegetarian hard cheese
• finely grated
• salt and freshly ground black pepper

Directions

• For the filling
• melt the butter in a wide lidded saucepan over a medium–high heat. Add the leeks and onion and fry for a few minutes. Cover
• lower the heat and cook for 15 minutes
• or until tender.
• Meanwhile
• bring a pot of salted water to the boil. Boil the potatoes for 8–10 minutes
• or until just tender
• then drain.
• Gently warm the milk.
• Remove the lid from the leeks
• turn up the heat and cook off any excess liquid. Sprinkle in the flour
• stir and cook for 1 minute. Gradually pour in the milk
• stirring all the time
• and bring to the boil until thickened. Add the mustard
• thyme
• cheeses and potato. Season with salt and pepper.
• Pour the mixture into a 25cm/10in pie dish and sit a pie funnel in the middle. Set aside to cool.
• To make the pastry
• put the flour into a bowl. Coarsely grate the butter and vegetable fat into the flour. Add 150ml/5fl oz cold water and stir to make a firm dough. Sprinkle the work surface with flour. Knead the dough
• then roll it out into a rectangle. Fold the bottom third of the rectangle up into the centre
• and fold the top third down
• like a letter. Turn 90 degrees
• roll out to a rectangle and fold in same way again. Turn 90 degrees and fold again
• then repeat. Chill in the fridge for 30 minutes.
• Preheat the oven to 220C/200C Fan/Gas 7. Roll out the pastry and cut off 1cm/½in-wide strip long enough to fit around the circumference of the pie dish (cut two or more strips if needed for the length). Roll the remaining pastry into a circle about 3cm/1¼in bigger than the pie dish. Wet the lip of the pie dish
• lay the thin strip of pastry on top and press down. Wet the top of the pastry lip and lay the circle on top
• press down and crimp the edge. Make a steam hole to expose the funnel.
• Roll the pastry trimmings and cut leaf shapes for decoration. Brush the pastry lid with egg wash
• arrange the leaves on top and brush those with egg wash too.
• Put the pie dish on a baking tray and bake for 40–45 minutes
• or until golden-brown and crisp and the filling is piping hot. Serve with green vegetables.