Preheat oven to 170°C/Fan 150°C/Gas 4.
Peel celeriac and cut in half and half again. Slice thinly - about 1mm thick - or use a mandoline if you have one.
Place in a pan with milk, cream, garlic, nutmeg and bring to a simmer. Cover and leave to cook for five minutes.
Remove from heat and lift celeriac with slotted spoon into a baking dish, arranging the slices on top of each other. Pour liquid over the top, cover with grated cheese and bake in the oven for 30 minutes until top is nicely browned. Serve with ham, or cold turkey.