Coat a deep, overproof dish wtih softened butter and reserve in the refrigerator.
Combine the milk, freshly grated nutmeg, bouquet garni, and garlic in a medium saucepan over medium heat. Bring the liquid to a boil, remove it from the heat, and leave it to infuse for 5 minutes.
Meanwhile, wash and peel the potatoes and slice them thinly, about 1/8". Do not rinse the potatoes as the starch is needed. Layer the potato slices tightly in the buttered dish and season each new layer with salt and pepper.
Stir the cream into the hot milk then pour through a fine mesh sieve onto the potatoes. Let rest for 1 to 2 minutes. The potatoes will absorb the liquid and the level will decrease. Top up with the remaining liquid. Repeat until all the milk mixture is used. Season the top and sprinkle generously with grated cheese. Cover the dish with buttered parchment paper and foil.
Place it in the over to cook for 1 hour or until the potatoes are easily pierced with a knife.
Remove the foil and parchment paper and return the dish to the oven to brown until the surface is golden.
Remove from the oven and let rest for 5 to 10 minutes before serving.