Heat the milk in a large pan until just simmering. Gradually stir in the cheese until melted. Mix the cornflour with 1 tbsp water then add to the pan and bubble up gently until thickened. Season with salt and white pepper, then pour into a clingfilm-lined, round 18cm solid cake tin or baking dish. Put in the fridge for 2 hours or until completely chilled.

Heat the oven to 220C/fan 200C/gas 7. Halve the pastry and roll into two 23cm circles. Put one piece on a baking sheet and spread over the mustard in an 18cm circle. Lay the ham on the mustard. Flip the set cheese out of its tin, remove the clingfilm and sit it on the ham. Brush the border of the pastry with egg then drape the second piece of pastry on top. Press around the cheese and ham, eliminating any air. Trim the edges then crimp to seal. Brush all over with some egg then put in the fridge for 10 minutes. Take the galette out and use the back of a knife to score a border 11/2cm in from the edge, then score lines from the centre to the border to create a pattern. Glaze with egg again.

Bake for 10 minutes then turn the oven down to 180C/fan 160C/gas 4 and cook for 20 minutes or until puffed and golden. Leave for 10 minutes before cutting into wedges.