Preheat the oven to 400°F (200°C) mark 6.

Cut off the hard, woody bases and the leaves of the celery bunches, separate the stalks and run a potato peeler down the outside of each stalk to get rid of any strings. Cut the stalks into 1 1/2 in (4 cm) lengths, place in a wide, fairly shallow saucepan or frying pan, add the stock, cover tightly and cook for about 7 minutes or until tender but still crisp.

Drain off the liquid into a small saucepan and add the shallot and thyme; boil until the liquid has reduced by half. Mix the cornflour or potato flour with a little cold water, stir into the liquid and then simmer for 1 minute while stirring the slightly thickened liquid.

Grease a gratin dish with butter and spread out the celery in a single layer.

Melt 1 1/2 oz (40 g) butter in a small saucepan and sprinkle it all over the celery. Sprinkle with the grated cheeses and a little freshly ground pepper. Drizzle the slightly thickened stock all over and place in the oven for about 10 minutes.