Wash the salsify thoroughly and scrape clean. To keep the sticks white, immediately place them in 1 liter of cold water mixed with 1-2 tablespoons of vinegar and 1 tablespoon of flour. Cut the salsify into 3 cm long pieces.
Heat the water with a little salt, add the salsify, bring to the boil, cook over low heat for 40-50 minutes and remove from the broth.
Melt the fat over high heat and heat the flour in it, stirring, until it is light yellow. Gradually add the salsify water, bring to the boil and let it steep for 10 minutes over low heat. Season the soup with salt, peel off with the beaten egg yolk and add the pieces of black salsify.