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Heat the oven to 160C/140C fan/gas 3. Brush the oil over the bottom and sides of a 25 x 19 x 5cm ovenproof dish. Put the milk, garlic, 4 thyme sprigs, bay leaf and shallot into a saucepan. Bring just to the boil, then remove from the heat and add the nutmeg and a little pepper. Set aside to infuse.
Peel then slice the potatoes very thinly. Pat them dry. Layer half the slices in the dish, overlapping slightly, then season.
Strain the infused milk into a jug. Put the crème fraîche into a bowl, then gradually pour in the milk, whisking until smooth. Add the stock and thyme leaves. Return this liquid to the jug, then pour half of it over the potatoes in the dish. Layer up the rest of the potatoes, then add remaining liquid, the cheese and a grind of pepper. Sit the dish on a baking sheet and bake for 1¼-1½ hrs until golden and tender – test by inserting a sharp knife through the potatoes. Stand for 5 mins before serving, sprinkled with thyme sprigs.