Chop the onions and sauté in butter with the bay leaf and the allspice. Add the mushrooms, ground caraway and salt, pour in the stock and stew for about 30 minutes. Mix the sour cream with half the milk and thicken the mushrooms. Cook for 10 minutes, add the remaining milk with the beaten egg yolk. Cook for one minute, add the dill and remove from the heat. Add lemon juice, dill and sugar according to your taste. Serve with pasta or potatoes; it is also excellent with fresh bread.