Put the dumplings in a pot of cold, slightly salted water. Bring everything to the boil and let it simmer at low heat for about 15 minutes. Clean the mushrooms, wash if necessary and let them drip off on kitchen paper.
Cut the mushrooms in half. Heat oil in a pan. Sauté the mushrooms in it in portions and take them out. Peel and finely dice the onions and sauté in the frying fat. Add the mushrooms. Season with salt and pepper.
Wash the thyme, chop it coarsely and add it to the mushrooms, except for something to garnish. Deglaze with broth and cook for 2 minutes. Stir in crème légère and mustard. Remove the dumplings, remove from the bags and arrange on plates with the mushroom pan.
Garnish with thyme and parsley.