Wash, peel and thinly slice the potatoes. Peel the onion and cut into cubes. Grease a suitable microwave dish (or glass dish that fits into the micro) with the butter. Put in a layer of potatoes in a ring. Sprinkle with the diced onion, pepper and salt. Repeat this until the potatoes are used up. Release the middle. Pour the cream over it, sprinkle with the cheese and marjoram. Close the pan and cook for 10 minutes at 600 watts. Cook for another 5 minutes at 360 watts. Then let stand for 5 minutes. Serve with a salad.