While the potatoes are cooking, peel the onion and slice it finely.Do the same with the leek white. In a pan, gently fry them with the butter until tender, about 10 to 15 minutes. Salt, pepper and stir from time to time.
Pour in the cream, add a pinch of grated nutmeg and a Saint-Marcellin cut into pieces.
Heat for 3 to 5 minutes over low heat, stirring, until the cheese is melted.