Place the dumplings in boiling salted water. Take the pot from the stove and let the dumplings simmer for about 5 minutes. Take out dumplings and rinse with cold water. Remove dumplings from the cooking bag and let them cool down.
In the meantime peel and finely dice the onion. Clean and wash the leek and mushrooms. Cut leek into rings. Halve the mushrooms if necessary. Heat 1 tablespoon of oil in a pot and fry the onion in it.
Add leek and mushrooms and fry briefly. Season with salt, pepper and nutmeg. Deglaze with stock, wine and cream and cook for 6-8 minutes at low heat. Bind with sauce thickener and season again.
Quarter dumplings. Heat the rest of the oil in a frying pan and fry the dumplings all around until golden brown. Clean, wash, quarter and seed the tomatoes. Cut into small cubes. Arrange dumplings and leek-mushroom vegetables on plates, sprinkle with diced tomatoes and serve garnished with parsley as desired.