In a tall stockpot, bring all the ingredients except the salt, pepper, and butter to a boil. Lower the heat and simmer for 30 minutes, or until each vegetable is easily pierced by the point of a knife. Pass soup through the medium disc of a food mill. Or skim the vegetables out of the liquid, pulse to a coarse puree in a food processor, and recombine the vegetables with the liquid. Season with salt and pepper. The soup can be refrigerated at this point for up to 4 days or frozen for up to 2 months. Bring the soup to a simmer. Stir in the butter just before serving.
Recipe courtesy of Barbara Kafka, "Soups: A Way of Life"