From "Splendid Soups," by James Peterson (Bantam Books, 1993, $29.95).
Cook beets, onion and marjoram in butter in a 4-quart pot over medium heat until onion begins to soften slightly, about 10 minutes. Add broth, partially cover pot, and simmer slowly for about 30 minutes, until beets are completely soft.
Check them by trying to crush one against the side of the pot with a wooden spoon. Simmer longer if necessary.
Puree soup in a blender or food processor. If you want soup to have a smoother texture, strain it through a medium-mesh strainer. Add cream of vinegar and bring soup back to a simmer. Season with salt and pepper.
To serve, ladle into bowls and garnish with whipped cream, croutons and herbs, or serve garnishes separately and let diners help themselves.