For the pie filling

• 60g (½ stick, 4 tablespoons) butter

• 1 large leek, sliced into half lengthways and then thinly sliced
• 
1 large onion, quartered and thinly sliced
• 
800g (1.7Ib) potatoes, cut into ¾-inch (2cm) cubes

• 600ml (2½ cups) whole milk

• 50g (5 tablespoons) all-purpose flour

• 1 tablespoon old fashioned Dijon mustard
• 
½-1 teaspoon dried thyme
• 
75g (3oz) mature Cheddar, grated (about 1 cup grated)

• 50g (2oz) Parmesan cheese, grated
• salt and freshly ground black pepper to taste

Ingredients for Flaky pastry

• 350g (12oz, 3 cups minus 2 tbsp) all-purpose flour
• ½ teaspoon salt
• 250g (1 cup +1 tablespoon, 9oz ) cold unsalted butter, diced into ½-inch cubes

• 150ml (5 fl oz, ½ cup + 2 tablespoons) cold water 
• 1 small egg beaten, for egg wash

Instructions

1. Preheat the oven to 425ºF (220ºC) Gas 7, ten minutes before using. You will need a  an 11 x 9-inch (28 x 22cm) oven dish or 12-inch (30cm) pie dish.
2. Boil the potatoes: bring a pot of salted water to the boil. Boil the potatoes for 8–10 minutes, or until just tender, drain and set aside.
3. To make the filling: melt the butter in large heavy based saucepan over a medium–high heat. Add the leek, onion, a little salt and pepper, and fry for a few minutes. Cover the saucepan, lower the heat and cook for 15 minutes, or until tender. Sprinkle the flour over the leek and onion mixture and cook stirring for 1 minute. Gradually pour in the milk, stirring briskly until the mixture comes to the boil. Continue stirring, 1-2 minutes until thickened. Switch off the heat, add the cheeses and stir until melted. Season with salt and pepper, then add the mustard, thyme, and potato and stir gently to combine. Transfer the mixture into the prepared pie dish and set aside to cool.
4. Make the pastry:. put the flour and salt into the bowl of a food processor. Add the cubes of butter and pulse a few times until the butter resembles large peas. Gradually add the cold water and pulse until the dough starts coming together then stop. Turn the contents onto the counter and bind everything into a dough.
5. Roll the dough and fold three times: shape the dough into a small oblong, flour the surface and roll it into a rectangle. Fold the bottom third of the rectangle up into the centre, and fold the top third down, like a book. Turn 90 degrees, roll out to a rectangle for the second time and fold in same way again. Turn 90 degrees and fold again, then roll into a rectangle the third time. Fold into a book and chill in the fridge for at least 30 minutes.
6. Roll the pastry: roll out the pastry into a large rectangle and cut off  a 1cm/½-inch wide strip along the edge, to fit the circumference of the pie dish or the rim of the oven dish. Brush the lip of the oven dish with egg wash and lay the ½-inch strip of pastry along the lip and press down firmly. Brush the strip of pastry with egg wash and lay the rolled pastry on top, press down, cut off excess pastry and crimp the edge. Using a sharp scissor cut a cross in the middle and make a small funnel using aluminium foil wrappep over your finger and place the funnel in the middle. Roll the pastry trimmings and cut leaf shapes for decoration. Brush the pastry with egg wash, arrange the leaves on top and brush those with egg wash too.
7. Bake the pie: place the pie dish on a baking sheet and bake for 40–45 minutes, or until golden-brown and crisp and the filling is piping hot. Serve with green vegetables.
8. For cup measures: spoon the flour into the cup, heaping it up over the top, then slide a knife across the top to level off the extra. Be careful not to shake or tap the cup to settle down the flour or you will have more than you need. I use a 240ml American cup.

Points to Note

• Please note that oven temperatures are given as a guideline only. You may need to add or reduce the suggested temperature depending on your oven. I baked the pie at 425°F (220°C) for 45 minutes. 
• You can make the filling a day in advance and keep it covered in the fridge. You can also roll the pastry up to 6 hours in advance and keep the dish in the fridge, uncovered.
• The pastry weighed 735g (26oz, 1.7 pound) and I used 400g (14oz). I had 300g (10oz) left over that I used to make 3 chicken pot pies.
• Last Updated: January 3, 2019

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