Cook the chopped scallions in the margarine until soft. Add the chopped cucumbers and the wine vinegar. Pour in the broth, and stir in the farina. Salt to taste, and add tarragon. Let the soup simmer for 20 minutes, or until the cucumbers are soft.
Put the soup into a blender, and puree it.
Pour the puree into a bowl, and whisk in the sour cream. Taste the soup for seasoning. Pour the soup into bowls, and garnish with cucumber slices and chopped parsley.