Preheat the oven to 180 C (356 F).

Grease the gratin dish with butter.

Peel the sweet potatoes and cut them into thin slices using a vegetable mandoline.

Arrange the potato slices in the dish in overlapping rows, 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 the salt, pepper and 5 sage leaves, then cover and let the cream infuse for 5-10 minutes.

Pour the infused cream over the potatoes in the dish, then cover it with a lid or foil.

Bake the covered gratin in the oven for 40 minutes.

After 40 minutes, remove the gratin from the oven, sprinkle it evenly with the grated Gruyere and return it to the oven for another 10 minutes, or until the cheese melts and turns golden brown.

Let the gratin cool slightly before serving.