Preheat the oven to gas 2, 150 C, fan 130 C. Combine the cream, milk, garlic, sage and mustard in a pan. Heat until just coming to the boil, then remove and season with a little freshly grated nutmeg. Discard the sage.
Meanwhile, blanch the kale for 1 minute in a pan of gently simmering water, then drain and refresh in cold water. Drain again, then dry with kitchen paper.
Grease a baking dish with butter. Pour in a little of the cream mixture. Top with a layer of celeriac and potato, season, then top with a little kale. Repeat until all the ingredients are used up, finishing with a layer of the cream mixture. Cover with foil and bake and 1 hour and 40 minutes.
Heat the grill to medium. Remove the foil and finish off under the grill until golden.