Clean the leek, cut in half lengthwise, wash and dab dry. Heat butter in a large pan. Sauté the leek sticks briefly all around. Peel and wash the potatoes and cut or slice them thinly. Quarter apples, remove seeds and also cut into thin slices. Layer potatoes, apples and ham in 4 smaller or 2 larger gratin dishes like roof tiles
Whisk eggs, cream and milk. Season with salt, pepper and nutmeg. Pour egg cream over the layered ingredients. Place leek on top. Coarsely grate the mountain cheese and sprinkle over the leek. Close each mould with aluminium foil. Bake gratins in a preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: see manufacturer) for approx. 20 minutes. Remove foil. Bake for another 25 minutes
Meanwhile, roast the hazelnut leaves in a pan without fat until golden brown, take them out. Sprinkle the finished gratins on top