Cut the skin off the celeriac. Chop into quarters. Thinly slice. Peel and slice the potatoes as thinly as you can. Put the sliced vegetables into a bowl of cold water to stop them from discolouring
Rub some butter around the bottom and sides of the baking dish
Layer the bottom of the dish with a mix of potato and celeriac slices. Pour some cheese sauce over
Repeat the process with another layer of sliced vegetables and cheese sauce until the dish is full
Put the dish in the middle of a preheated warm to medium high oven for about an hour. The gratin should be browned on top and cooked all the way through - test with a small knife