Step 1
Preheat the oven to 200°C, fan oven 180°C, Gas Mark 6. Lightly grease 2 baking sheets with a little Trex.

Step 2
Sift the flour, salt and mustard powder (if using) into a mixing bowl and rub in the Trex with your fingertips until the mixture looks like fine crumbs. Stir the grated cheese into the mixture, then add just enough chilled water to make a smooth, but not sticky dough. Wrap and chill for 10-15 minutes.

Step 3
Meanwhile, simmer the leek and carrot in lightly salted water for 10 minutes, until just tender. Drain and leave to cool.

Step 4
Roll out the pastry on a lightly floured surface to form a large rectangle. Use a saucer as a template to cut out six circles measuring 15cm (6in) in diameter, re-rolling the pastry as necessary.

Step 5
Mix the cooled vegetables with the ham, diced cheese and herbs. Season with black pepper. Share between the pastry circles, leaving a border around the edge. Brush the border with beaten egg, then fold the pastry over to make semi-circles. Seal the pastry, crimping the edges together.

Step 6
Arrange the pasties on the baking sheets and brush with beaten egg. Bake for 25-30 minutes until golden brown. Cool for a few minutes, then serve.