Grill bacon under medium grill for 10 minutes then slice into strips.
Interleave the celeriac, potatoes and a third of the grated cheese in an ovenproof dish.
Lay most of the bacon (you need to keep some for garnishing) on top and pour over cream. Sprinkle the rest of the cheese over the top and add rosemary. Place in the centre of a preheated oven for 40 mins at 190 C / Gas mark 5.
When cooked garnish with bacon and serve.
The bacon is optional, so this dish could be suitable for vegetarians.