Brush the black salsifies vigorously under running water, peel, wash again and place in slightly acidified vinegar water so that the vegetables do not turn brown. Cut the black salsifies into thick, diagonal slices and cook for 15 minutes in lightly salted, slightly boiling water.
In the meantime, heat the fat for the sauce in a saucepan and sauté the flour in it until golden brown. Deglaze with stock and milk and simmer for a few minutes, stirring. Wash the parsley and chop it except for something to garnish.
Stir the egg yolks into the hot sauce (do not boil any more!) and add the chopped parsley. Season to taste with mustard, salt and some lemon juice. Add the drained black salsify to the hot sauce and let it stand for a while.
In the meantime, heat the oil in a pan and fry the sausages for three to four minutes until crispy brown, turning them over. Arrange everything on plates garnished with parsley. Boiled potatoes taste good with it.