Peel and wash the potatoes and rutabaga and cut into very thin slices. Peel and finely chop the onion. Peel garlic and chop finely. Wash thyme, dab dry and pluck leaves from the stalks.
Heat the oil in a small pot. Sauté onions and garlic in it until transparent, add thyme and then deglaze with stock and cream. Bring to the boil and season vigorously with salt and pepper. Layer turnip and potatoes in an ovenproof, greased dish.
Pour cream over it, sprinkle with cheese and bake in the preheated oven (electric oven: 200 °C/ gas: level 3) for 40-50 minutes. Then remove from the oven and serve garnished with thyme and marjoram as desired.