Put the dumplings in a pot with plenty of cold water and let them swell for 10 minutes. Bring to the boil briefly and let it simmer for 10 minutes at low heat. Cut the sausages into slices. Clean and wash the leek and cut into rings.
Clean and halve the mushrooms. Heat the oil. Sauté the mushrooms well in the oil, fry for about 5 minutes at medium heat, remove. Put the sausages in the pan and fry them. Add leek, pour 1/4 litre of water and milk.
Bring to the boil, stir in mustard and sauce thickener, season with salt and pepper. Add mushrooms, heat briefly. Wash parsley, dab dry and chop except for a little bit for garnishing. Remove the dumplings from the water and quench.
Tear open the cooking bag and take out the dumplings. Arrange the dumplings with the sausage ragout on plates. Serve sprinkled with parsley and garnished.