Wash the kale thoroughly, drain and roughly cut. Peel and chop the onions. Heat the oil in a pot. Brown the onions in it. Add kale and 1/4 litre stock and braise at low heat for about 45 minutes.
Peel, wash and cut the potatoes into pieces. Boil up the remaining stock and mustard seeds. Add potatoes and cook for about 15 minutes. Then add the broth to the kale. Prick the sausages with a fork, add them to the kale and cook for about 5 minutes.
Season the kale stew with salt and pepper. Sausages may be cut in half.