Peel the potatoes, wash and dab dry. Cut potatoes into 3-4 mm thick slices. Remove the skin from the sausage. Cut sausage into approx. 5 mm thick slices. Put the potato and sausage slices in a deep baking tray (25x35 cm; approx. 3 1/2 litres capacity). Peel onions and cut into strips. Heat oil in a pot and sauté the onion in it.
Peel the garlic, press it through a garlic press and add it to the onions. Add milk, cream and sour cream and bring to the boil. Stir in mustard and season with salt and nutmeg. Pour onion-cream mixture over the potatoes. Coarsely grate the cheese, spread on the casserole and bake in the preheated oven (electric range: 175 °C/ gas: level 2) for approx. 1 hour on the 2nd rack from below. Garnish as desired with fresh marjoram
With 10 people: