Peel and chop the shallots. Rinse the black salsify to remove dirt. Peel and chop the salsify and Jerusalem artichokes and put them into a bowl of cold water. (Cold water prevents the roots from browning). Chop the leek. Sauté the shallots, salsify, and leek in butter in a pan for a few minutes. Add the white wine and cook, stirring frequently, for a minute or so. Pour in the vegetable broth. Bring the mixture to boil and let simmer under a lid for about 20 minutes or until the vegetables are cooked through. The cooking time will depend on the size of the vegetable chunks. Whizz the soup using a hand blender. Add the cream. If the soup is too thick, add more broth. Season the soup with white pepper and salt. Salt may not be necessary as the vegetable broth has salt. Ladle into bowls and scatter some seeds on top.