Peel the potatoes and cut into 2-3 mm thin slices. Peel and quarter the turnip and cut into 2-3 mm thin slices.
Grease a flat baking dish with butter, add the turnip and potatoes alternately like a roof tile. Season with salt and pepper.
Bring the cream to the boil, season with salt, pepper and nutmeg and reduce for 5 minutes over a mild heat. Pour the cream into the mold. Bake in a preheated oven at 180 ° C for 35-40 minutes.
Peel and chop the onion and apple and fry until golden brown in a pan. Add marjoram. Remove the gratin after 20 minutes and spread the apple and onion mixture on top. Sprinkle with sesame seeds. Cook the casserole dish in the oven for 10-15 minutes.