Heat the oven to 180 C (365F).

Grease the gratin casserole with butter.

Cut the peeled potatoes into slices using a vegetable mandoline.

Put the potato slices in the casserole, arrange them like a ratatouille.

Pour the cream into a small saucepan and heat it over low heat, until bubbles appear on the surface, and the cream begins to boil. Remove the saucepan from the heat, add salt and pepper, add 5 sage leaves, cover and let the cream infuse for 5-10 minutes.

Pour the cream mixture over the potatoes in the casserole. Cover the casserole with a lid or foil.

Put the casserole in the oven for 40 minutes.

After 40 minutes, remove the casserole from the oven, sprinkle with Gruyere cheese and put it back in for another 10 minutes or until the cheese melts and turns brown.

Let it cool before serving.