Potato and Celeriac Gratin

• 900 g (2 lbs) potatoes, thinly sliced
• 1 large celeriac, peeled, quartered and thinly sliced

Butter a gratin dish and layer with the potatoes and celeriac. Season each layer with salt and pepper and dot with butter. End with a layer of potatoes dotted with butter. Pour over the stock or cream and bake in a moderate oven 180°C/350°F/gas 4 for about 1 hour