Preheat the oven to 200C/400F/Gas 6.
Place all the ingredients except the cheese into a large saucepan and heat until the mixture begins to boil.
Pour the mixture into a small buttered gratin dish. Add the cheese to the top of the mixture and bake in the oven for 15 minutes or until bubbling, golden-brown on top and the celeriac is soft.
To serve, remove from the oven and spoon the gratin onto a clean plate.