Cut the rolls into small slices and put them in a large enough bowl. Boil 200 ml of milk and pour over the slices, put a lid on the baking dish and soak the rolls for a few minutes.
Clean and finely dice the leek. Sauté the onion cubes in a little bit of butter, stir in 1 tbsp leek cubes and then add half of the onion mixture to the soaked bread rolls form. Stir in 2 eggs and season the mixture with salt, pepper and a pinch of nutmeg.
Mix the remaining onions well with egg, mince, mustard, parsley and thyme. Season with pepper and salt.
Form dumplings from the bun mixture, make a hole with your thumb, fill in minced meat mixture and close with a small bit of bun mixture. Cook the dumplings in boiling salted water for 15 minutes.
For the sauce, sauté remaining leek in a saucepan with butter. Dust with flour, stir in remaining milk with a whisk and let it bubble up while stirring.
Stir in vegetable soup extract and season the sauce strongly with salt and freshly ground pepper.
Serve the dumplings with the sauce.