2 small leeks, thinly sliced
1 onion, thinly sliced
450g/1lb potatoes, cut into 2cm/¾in cubes
1 tbsp chopped fresh thyme
1 tbsp Dijon mustard
350g/12oz plain flour, plus extra for dusting
salt and freshly ground black pepper
50²µ/1¾´Ç³ú plain flour
125²µ/4½´Ç³ú butter, frozen for 1 hour
60²µ/2¼´Ç³ú butter
50²µ/1¾´Ç³ú mature cheddar, finely grated
600ml/20fl oz full-fat milk
50²µ/1¾´Ç³ú Parmesan or vegetarian hard cheese, finely grated
125²µ/4½´Ç³ú hard vegetable fat, such as Trex, frozen for 1 hour
1 free-range egg, beaten for egg wash