Clean the leek, wash thoroughly and cut into rings. Pre-cook in little boiling salted water for about 1 minute. Quench, drain. Cut 200 g cheese into slices. Cut ham and cheese into pieces of about 4 x 4 cm. Peel and wash the potatoes and cut them into thin slices. Peel and finely dice onion.
Grease casserole dish and sprinkle with onion. Layer potatoes, leek, ham and cheese like roof tiles. Season cream with salt, pepper and nutmeg and pour over potatoes. Grate 50 g cheese and sprinkle over the casserole. Cook in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: level 2) for approx. 50 minutes. At the end of the cooking time, cover with aluminium foil as required. Garnish with marjoram and serve