Preheat oven to 180˚C.
Firstly, butter your gratin dish. Slice the celeriac and potatoes thinly in your food processor or on a mandolin.
Heat the milk and cream together.
Layer the celeriac and potato in the gratin dish, seasoning with salt, pepper and thyme between layers and adding a soup ladle of the warm cream mixture.
Finish with a layer of potato and top with grated Gruyère. Bake for 40 minutes or until the celeriac and potatoes are soft.