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Squash the peeled garlic cloves with the blade of a knife, then put in a large saucepan with the cream, milk, onion slices, bay leaf and 2 tsp sea salt. Bring to the boil over a low heat. When the mixture is bubbling, remove from the heat and leave to infuse for 1 hr.
Heat the oven to 160C/140C fan/gas 3, and generously butter a large, deep baking dish. Sieve the infused milk over a bowl and discard the onion, bay leaf and garlic (you can reserve these to use in a bread sauce, if you like). Arrange a layer of the potatoes in the dish, then scatter over some of the chestnuts, sprouts, cheese and a little nutmeg. Drizzle over some of the infused milk mixture. Repeat until you’ve used all the ingredients, finishing with a layer of potatoes, then cover with the last of the infused milk mixture. Dot over a few small knobs of butter.
Bake the gratin for 1 hr 15 mins on the middle shelf of the oven until golden and crisp on top. A knife inserted into the middle should slide in easily. Leave the gratin to rest for 10 mins, then serve.