Replace the celeriac with two peeled and sliced sweet potatoes and use a strong Swiss cheese.
Heat oven to 190C/fan 170C/gas 5. Peel, quarter and
very thinly slice the celeriac - this is best done with
the slicing blade of a food processor. Toss in the
walnut oil, salt and pepper, and spread over a gratin
dish. Pour over the stock and bake for 40-45 mins,
then sprinkle with the walnuts, crumble over the
cheese, then cook for 15 mins until crisp and golden.