Step 1

Heat the oven to 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 celeriac 1-2mm thick (or slice carefully by hand with a knife). The celeriac is tricky to handle due to its size and shape, so cut it into half or quarters first for easier slicing.

Step 2

Arrange a layer of sliced celeriac 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.

Step 3

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.

Step 4

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 celeriac 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.