Wash the potatoes and cook covered for about 20 minutes. Quench, peel and let cool
Clean, wash and slice the mushrooms. Wash the thyme and, except for something to garnish, pluck it. Peel and chop the onion. Fry in 2 tbsp. hot oil. Add mushrooms. Fry over high heat for about 10 minutes until all the liquid has evaporated. Season with thyme, salt and pepper
Whisk eggs and milk, season with salt, pepper and nutmeg. Grate cheese. Cut potatoes into slices
Place the potatoes, mushrooms, ham and 2/3 cheese alternately in a large oiled casserole dish, pouring a little egg milk over each layer. Finish with potatoes and sprinkle with the rest of the cheese. Bake in the preheated oven (electric oven: 175 °C/ convection oven: 150 °C/gas: level 2) for 35-40 minutes. Garnish with the rest of thyme
Drink: cold beer