Preheat the oven to 180°C fan/200°C/ gas 6. Butter a medium-sized roasting tin or gratin dish, then add a layer of potatoes, followed by onions and mushrooms.
Season with a generous pinch of sea salt, black pepper and a grating of nutmeg, then repeat the layers, seasoning between each, finishing with a layer of potatoes.
Mix the garlic with the double cream, another good pinch of salt, pepper and grated nutmeg, then pour it all over the potatoes. Top with the reblochon, scatter over the sage leaves, then transfer to the oven and cook for 1 hour.
Let the gratin sit for 10 minutes, then serve with a crisp green salad alongside.