Butter a large ovenproof dish. Peel and slice the potatoes into very thin rounds.
Put them dry with paper towels, then arrange them evenly in the prepared dish.Season with salt, pepper and a little nutmeg if you like.
Crumble the blue cheese with a fork, then mix it with the cream and milk until well combined.
Pour the mixture over the potatoes, making sure everything is evenly coated.
Bake in the oven from 1h to 1h30, until the top is golden brown and the potatoes are tender when pierced with a knife. Put the dish under the grill for a few minutes to ensure the gratin is perfectly golden.