Peel onion and garlic. Cut the onion into strips and chop the garlic finely. Melt butter in a pot. Fry onion and garlic in it until transparent. Add apple juice, stock and cream, bring to the boil and simmer at low heat for about 5 minutes.
Wash the marjoram, shake dry and chop finely, except for something to garnish. Add chopped marjoram to the cream mixture, except for a little bit, and season to taste with salt, pepper and nutmeg. Remove the pot from the stove.
Peel, wash and slice the turnip and potatoes finely. Remove the core of the apples with an apple corer. Slice the apples into fine slices as well. Layer the vegetable and apple slices in a round casserole dish (24 cm Ø).
Season each layer with salt and pepper. Arrange the potato and apple slices in a fan-shaped arrangement as the top layer. Pour the cream mixture evenly over them and bake in a preheated oven (electric cooker: 175 °C/ circulating air: 150 °C/ gas: see
manufacturer) bake for about 1 hour. If necessary, cover with aluminium foil after approx. 50 minutes. Remove aluminium foil again. Crumble the cheese over the gratin and bake for another 10-15 minutes. Remove from the oven and garnish with the remaining marjoram.