400g/14oz celeriac, peeled and very thinly sliced (ideally with a mandolin)
2 smoked garlic cloves (or regular garlic if smoked unavailable), crushed
400g/14oz Maris Piper potatoes, peeled and very thinly sliced (ideally with a mandolin)
1 tbsp finely chopped fresh rosemary
1 tbsp Dijon mustard
salt and freshly ground black pepper
butter, for greasing
500ml/18fl oz double cream
100²µ/3Β½΄Η³ϊ Parmesan (or a similar vegetarian hard cheese), grated