Place celeriac in a saucepan; pour in enough heavy cream and milk to cover 3/4 the celeriac. Sprinkle with nutmeg. Bring celeriac mixture to a boil; reduce heat and simmer until celeriac is slightly tender, about 5 minutes. Transfer celeriac mixture to a 9x9-inch square baking dish; top with Cheddar cheese.
Bake in the preheated oven until bubbling and cheese is melted and browned, about 20 minutes.
