Preheat the oven to 110C/225°F.
Pour the milk into a saucepan and add the herbs and crushed garlic. On a medium heat, bring to the boil and leave to simmer for 5 minutes.
Remove from the heat and leave to cool so that the milk infuses with the herbs and garlic. Once infused, strain to remove the herbs and pour the milk back into a large heavy-bottomed pan.
Slice the potatoes as thinly as possible. Add them to the milk, separating them as much as possible, and place the pan on a medium heat. Bring the liquid to a simmer, stirring occasionally to stop them from sticking. After a few minutes, when the milk has thickened, add the cream and butter and gently agitate the pan so that everything is mixed inches.
Add the salt, cayenne pepper and nutmeg. Be careful - cayenne pepper is strong stuff. This gratin will take more salt than expected, as the potatoes absorb quite a lot.
Pour the potatoes into a baking dish at least 5cm/2 inches deep and measuring about 25 x 15cm/10 x 6 inches, trying to get them as flat as possible. The potatoes should be covered by about ½cm of liquid. A little less is fine, but if there is more liquid than that, do not add it. Tightly cover the dish with kitchen foil and bake in the oven for 4 hours. Every 45 minutes or so, gently press down on the potatoes with the back of a wooden spoon.
Just before serving, flash the dish under a hot grill to gratinate it.