This pastry is well worth the effort – it’s mouthwateringly buttery and flaky, enclosing a rich and deliciously cheesy filling. You can make both the pastry and the filling in advance and chill them in the fridge until you’re ready to assemble and bake.
Make the pastry. Freeze the 360g portion of butter for at least 15 minutes, or until hard, then coarsely grate (it’s easiest to do this in a food processor). Divide the butter in half and put both halves back in the freezer.
Place the flour, salt and 180g of chilled, cubed butter into a bowl and rub it between your fingertips until it resembles fine breadcrumbs (you can do this in a food processor if you wish).
Add the ice-cold water, and stir it in using a round-bladed knife. Add a little more water, if needed, until you can bring the mixture together with your hands to a fairly firm dough. Then, shape it into a rectangle about 2cm thick.
Turn the rectangle so that a short end is closest to you. Sprinkle 1 portion of the frozen butter over the top two-thirds of the rectangle, then fold the bottom third over the middle third, and the top third down over that (as if you were folding a business letter).
Turn the dough through 90 degrees and roll it into a rectangle again. Add the remaining butter, as before, and make another letter fold. Roll and fold one last time, then wrap the dough in cling film and chill it while you make the filling.
Heat the oven to 200°C/180°C fan/400°F/Gas 6. Cut the potatoes into 5mm slices. Sprinkle them over a large baking tray with the leeks and onion and toss everything together with the olive oil, thyme, pepper and salt. Roast for about 20 minutes, stirring occasionally, until golden.
Meanwhile, heat a frying pan over a medium heat. Add the pancetta and fry for 4–5 minutes, until golden and crispy. Drain on a piece of kitchen paper and leave to cool.
Once the potatoes and leeks are cooked, transfer them to a large mixing bowl and leave to cool for 10 minutes, then refrigerate for 10–15 minutes. Once fully cold, add the cooked pancetta, double cream, Dijon mustard, chilli flakes and cheese. Season with pepper to taste.
Increase the oven temperature to 220°C/200°C fan/425°F/Gas 7. Divide the pastry into 8 equal pieces, then roll each into a 19–20cm disc. Using the 19cm side plate as a guide, trim the discs into neat 19cm circles, each about 5mm thick (re-roll the trimmings to make an extra pasty if you like).
Brush a little beaten egg yolk around the edge of one half of each pastry disc, then divide the filling equally between each disc (compress the filling slightly with your hand to squeeze more in). Fold the pastry over the filling, pressing the curved edges together. Press to seal, then crimp the edges between your finger and thumb.
Transfer the pasties to the lined baking sheets. Brush them all over with beaten egg yolk, then cut a few steam holes in the top of each to prevent them splitting. Chill the pasties in the fridge for 5 minutes, then bake for about 30–35 minutes, or until golden brown. Serve warm.