Celeriac is the Marmite of vegetables. With its hint of aniseed and rough texture, this root can be divisive but, when cooked, it takes on an irresistible sweetness and a distinctly velvety texture. I love it in this seasonal gratin, which is rich with garlicky mushrooms, melting onions and hearty potatoes. It’s like sinking into a bath of autumn, and just needs a crisp salad on the side. Celeriac, potato and mushroom gratin The addition of mushrooms to the winning combination of celeriac and potato turns this from a side dish into a main course. If you are meat eaters, try frying some pancetta at the same time as the mushrooms. Prep 25 min Cook 1 hr 30 min Serves 4-6 2-3 medium celeriac (about 500g), peeled, scrubbed and quartered 3 medium potatoes (about 600g), peeled A squeeze of lemon 80g butter 400g chestnut mushrooms, sliced 1 handful thyme sprigs, picked Salt and black pepper 4 garlic cloves, peeled and finely chopped 250ml double cream 500ml chicken or vegetable stock 2 large onions, peeled and sliced Nutmeg 75g grana padano or parmesan, grated Using a mandoline, cut the celeriac and potatoes into 3mm-thin slices, and put these in a bowl of cold water with a squeeze of lemon juice while you prepare the other ingredients. Put a large frying pan on a medium-high heat and, when hot, add half the butter, the mushrooms and half the thyme, season generously and saute, stirring regularly, for five minutes. Add half the garlic, saute for another six to seven minutes, until the mushrooms have bubbled off their liquid and are looking golden, then take off the heat. Heat the oven to 220C (200C fan)/425F/gas 7. Whisk the cream and stock in a bowl. Drain the celeriac and potato, then arrange a layer of them in the base of a wide, deep ovenproof dish. Top this with half the sliced onion and half the mushroom mixture, dot with a third of the remaining butter, half the remaining garlic and thyme and a light sprinkling of the cheese, and season generously with salt, pepper and grated nutmeg. Repeat with another layer of the root vegetables, followed by the onion, the last of the mushroom mix, the remaining garlic and thyme, the butter, cheese and seasoning, then top with a final layer of potatoes and celeriac. Dot with the last of the butter, then pour the stock and cream over the top. Press everything down into the liquid, scatter with the last of the cheese and bake for 65-75 minutes, until golden, and the root veg are tender to the tip of a knife. Serve with a crisp salad, or some greens, or with grilled pork chops or sausages. The simple flex This indulgent gratin is also good boulangère-style – that is, without cream and just the stock, which works well for vegans. Use oil rather than butter, omit the cheese and top the gratin with some breadcrumbs instead.] This article was amended on 14 November to clarify when to add the garlic and thyme