Peel and wash the potatoes and slice them into thin slices. Put them in four ovenproof serving pans in flakes. Season the cream with salt and pepper and pour over the cream. Bake in the preheated oven (electric cooker: 175 °C/ gas: level 2) for about 30 minutes.
Meanwhile clean the mushrooms, rub with kitchen paper (cut off shiitake stems) and cut in half. Pluck off the sage leaves, except for something to garnish. Wash the apple, quarter it, core it and cut it into slices.
Sprinkle with lemon juice. Fry the mushrooms in hot oil while turning for about 5 minutes, season and remove. Sauté apples in frying fat, season with sage. Fold in mushrooms. Arrange on the gratin. Garnish with sage leaves.