Peel and wash the potatoes and cut them into 2 cm (1 in) cubes.

Finely dice the onions and keep just under half the quantity to one side. Trim the leeks, wash thoroughly and then slice them.

Heat the oil in a large pan and fry the onions and leeks for 5 minutes, stirring from time to time. Add the potatoes and hot stock, cover the pan and simmer for 20 minutes.

Soften the roll in cold water, squeeze out excess moisture and then mix with the egg, minced meat, the remaining onion, the nutmeg, pepper and salt. When thoroughly mixed, shape the mixture into balls about the size of a walnut.

Drop the dumplings into the soup and simmer for about 10 minutes, until cooked through.

Cut the tomatoes into wedges and add them to the soup. When the tomatoes are cooked, season the soup with marjoram and celery salt and sprinkle with chopped parsley.