Wash the potatoes and cook in boiling water for about 20 minutes, drain, rinse and peel. Let the potatoes cool down and cut into slices. Melt the fat in a pot. Sweat flour in it and add milk while stirring. Add stock and bring to the boil.
Season with salt, pepper and nutmeg. Clean and wash the spinach and let it collapse briefly in boiling salted water. Pour onto a sieve and drain. Clean, wash and cut the tomatoes into thick slices. Grease a large ovenproof casserole dish. Put some béchamel sauce in the dish. Place a layer of potatoes on top. Then a layer of spinach, potatoes, tomatoes and ham and again potatoes. Pour some sauce over each layer. Pour the rest of the sauce over it.
Place a layer of potatoes on top. Then a layer of spinach, potatoes, tomatoes and ham and again potatoes. Pour some sauce over each layer. Pour the rest of the sauce over it. Grate cheese finely and sprinkle over potato lasagne. Sprinkle with marjoram. Bake in the preheated oven (electric cooker: 200 °C/ gas: level 3) for 40 minutes