Thoroughly brush the black salsifies under running water, peel and slice them diagonally. Immediately place in vinegar water. Peel, wash and coarsely dice the potatoes.
Peel and chop the onion. Heat 2 tablespoons of fat in a saucepan. Fry the onion until transparent. Drain black salsify and steam briefly with potatoes. 3⁄4 Pour on l water, bring to the boil and stir in the stock.
Cover and simmer for about 20 minutes.
Remove the crusts from toast and cut into fine cubes. Heat 1 tbsp. fat in a pan and roast the toast cubes in it until golden brown. Take them out.
Remove a few pieces of black salsify from the stock with a skimmer. Puree everything briefly. Add cream to the soup. Season soup with salt, pepper, nutmeg and lemon juice. Put the black salsify pieces back into the soup.
Arrange the soup in four deep plates and sprinkle with croutons. Garnish with parsley and cayenne pepper.