Wash the potatoes and cook in boiling salted water for 10-12 minutes. Drain, peel and slice. Grate cheese finely and mix with egg yolk and sour cream. Season with salt, pepper and some nutmeg.
Clean, wash and cut the leek into rings. Clean mushrooms, wash briefly and cut into slices. Layer everything in flakes in a greased casserole dish. Add cheese cream and sprinkle with hazelnuts.
Bake in the preheated oven (electric cooker: 175 °C/ gas: level 2) for about 40 minutes.