First make the pastry. Combine the flour and butter in a bowl and add enough cold water to bring the mixture together to form a rough dough.
Turn the pastry out onto a lightly floured surface and knead gently to form a rough ball. Flatten slightly then wrap in cling film and chill for one hour.
Roll the dough out to the thickness of a pound coin and cut out two 20cm discs. Lay a piece of pork on one edge of a pastry disc, cover with a piece of cheese then top with the ham.
Spoon some mustard over the top then repeat with the other pastry disc and filling.
Brush the edge of the pastry with egg yolk then fold over to seal. Pinch and fold the edges to crimp then transfer to a baking tray. Brush with egg yolk then chill for 30 minutes until firm.
Preheat the oven to 180 degrees Celsius or 350 degrees Fahrenheit and bake for 50 to 60 minutes until golden brown and crisp. Leave to rest for 10 minutes before serving.