Melt butter; add vegetables and salt. Stir, cover, and cook over very low heat until tender (30-40 min).
Puree vegetables, stock, and orange juice in blender or food processor very thoroughly.
Place large sieve over soup pot; strain mixture through it, stirring and pressing with back of a large spoon. Scrape underside periodically. In the end there will be a small amount of fibrous material in the sieve—discard it.
Heat soup to serving temperature. Thin with more stock if necessary. Season with sugar, salt and pepper.
