Three-root boulangère

Preheat the oven to 180°C/Gas Mark 4.

Add the oil or butter to a heavy-bottomed frying pan and use some of it to grease a large gratin dish.

Add the onions to the pan and sauté over a medium heat for about 10 minutes, until nice and soft, then add the garlic and cook gently for a further minute or two.

Meanwhile, peel the celeriac, potatoes and parsnips and cut into slices the thickness of a 10p piece, slicing the parsnips lengthways.

Spread out the celeriac in the gratin dish, season generously with salt and pepper, then sprinkle with half the onions and half the herbs.

Layer the parsnips on top, then scatter the remaining onions and herbs on top and finish with a layer of potatoes.

Bring the stock to a simmer and add some salt and pepper, then pour over the vegetables to barely cover them (you may not need all of it).

Cover the dish with foil and bake for 30 minutes, then uncover and continue to bake for another 30 minutes or so until the vegetables are cooked.

At this point, if there is still liquid covering the potatoes, spoon a little off the top and return the dish to the oven for 15 minutes or so, to brown the potatoes on the top.

You can use the grill in the oven to get a darker, crisper top if you like. Serve piping hot.