Heat the oven to 200°C. Liberally coat a gratin dish with half the butter, and rub with the cut surface of the clove of garlic.
Arrange half the potatoes over the bottom of the dish, season with salt and pepper and anoint with
Cover with the sliced mushrooms and sprinkle the powdered ceps over the top. Season with salt and plenty of pepper, coat with 2 more tablespoons of cream and cover with the remaining potatoes, laid on to overlap like the scales of a fish.
Season lightly, dot with the remaining butter and cover with foil. Cook for 45 minutes.
Remove the foil and bake a further 30 minutes. Turn up the heat to 220°C, coat the potatoes with the remaining cream and scatter the grated cheese over the top. Return to the oven for about 10–15 minutes, or until the top starts to fleck with golden patches.