2 small leeks, thinly sliced
1 onion, thinly sliced
450g/1lb potatoes, cut into 2cm/戮in cubes
1 tbsp chopped fresh thyme
1 tbsp Dijon mustard
350g/12oz plain flour, plus extra for dusting
salt and freshly ground black pepper
50驳/1戮辞锄 plain flour
125驳/4陆辞锄 butter, frozen for 1 hour
60驳/2录辞锄 butter
50驳/1戮辞锄 mature cheddar, finely grated
600ml/20fl oz full-fat milk
50驳/1戮辞锄 Parmesan or vegetarian hard cheese, finely grated
125驳/4陆辞锄 hard vegetable fat, such as Trex, frozen for 1 hour
1 free-range egg, beaten for egg wash