Marcus Wareing’s delicious twist on dauphinois includes celery root and tahini. Serve the creamy gratin as a main with a crisp salad, or as a side dish to go with roast meat.
Butter to grease
800g floury potatoes, peeled
1 large celery root (about 700g), peeled
2 onions, thinly sliced
4 thyme sprigs, leaves picked, plus extra to garnish
200g tahini
600ml good-quality vegetable stock, warmed
½ tsp fine sea salt
½ tsp ground white pepper
1 tsp garlic powder
Handful grated mature cheddar to cover
Heat the oven to 325F/160°C fan/gas 4 and grease a large casserole (about 26cm) with butter.
Using a mandoline or food processor with a slicer function, thinly slice the potatoes and celery root 1-2mm thick (or slice carefully by hand with a knife). The celery root is tricky to handle due to its size and shape, so cut it into half or quarters first for easier slicing.
Arrange a layer of sliced celery root in the bottom of the greased baking dish and season with a little of the salt and pepper. Follow with a thin layer of sliced onion and thyme leaves, then a single layer of potatoes. Season again. Repeat the layering of vegetables and thyme until they’ve all been used, ensuring the final layer of potato is arranged neatly as this will be visible when you serve the dish.
Whisk the tahini and vegetable stock together in a jug until evenly combined, then mix in the remaining salt and pepper and the garlic powder. Pour the tahini mixture over the layered vegetables, allowing it to sink in between the layers, then cover with foil.
Bake the gratin in the oven for 1¼-1½ hours. To test if it’s cooked, push a small knife into the centre – the potato and celery root should be soft all the way through and the knife should meet no resistance. If necessary, cover with foil again and return to the oven for 15 minutes more until soft (before adding the cheese).
When the gratin is cooked, remove the foil, sprinkle with grated cheese and return to the oven for 15 minutes to brown. Sprinkle with extra thyme and wait a few minutes before serving – it will be very hot.