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1
Preheat the oven to 180u00b0C
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2
Heat three-quarters of the butter in a saucepan, add the mushrooms and cook for 5-l0 minutes, until the mushrooms are tender, If there's any liquid, strain it off and save it.
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3
Slice the potatoes - which can be peeled or not, as you wish - as thinly as you can; (use a mandolin) Put the potato slices into a colander and rinse under the cold tap to get rid of the excess starch.
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4
Peel and chop the garlic and mix with the remaining butter Use this butter to grease a large shallow gratin dish generously. I use one that measures (9 x 12 in).
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5
Layer half the potatoes into the dish and season well with salt, pepper and nutmeg. Spoon the mushrooms on top, season again, then top with the remaining potatoes and season once more.
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6
Pour the cream over the top, then fill the cream carton with any liquid that you strained from the mushrooms, plus water to fill it to the top, and pour that over the potatoes, too.
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7
Drizzle the truffle oil over the top and bake for 1.5 hours, until you can easily insert a sharp knife into the potatoes and the top is golden brown. Part of the joy of this dish is the contrast between the crunchy top and the meltingly tender potato within. However, if the top seems to be getting too brown, cover it with some foil, but remove it 10-15 minutes before the end of cooking to crisp the top.
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8
You can make a vegan version of this dish using 600 ml (1 pint) of vegetable stock instead of the cream and water
