Peel the salsify, cut into large pieces and immediately put them in cold water mixed with flour and vinegar so that they do not discolour.
Clean the leek, cut into rings and sauté in the butter. Peel and dice the potatoes. Add the potato cubes and pieces of black salsify. Pour in the vegetable stock, season with marjoram, white pepper and nutmeg. Let simmer for about 25-30 minutes. Remove part of the soup, then puree and return. Add the cream.