Wash the potatoes and cook them in lightly salted water and then drain them. Let cool down a little.
Heat the butter in a saucepan, sieve in the flour and make a roux while stirring. Then peel and finely chop the onion and sauté in the roux. Pour in the vegetable stock and bring to the boil. Thicken while stirring constantly, add the cream and season the sauce with salt, pepper and sugar.
Then peel the potatoes and cut into approx. 1 cm thick slices. Mix with the sauce and season with the marjoram. Rub the marjoram well with your fingers.