1. Soak dried mushrooms,
2. Trim scallion stalks; slice ginger root and combine in a pan with canned shark's fin, sherry and water.
3. Bring to a boil, then simmer, covered, 15 minutes. Drain shark's fin, discarding liquid, scallions and ginger root.
4. Meanwhile skin, bone and shred chicken breast. Shred soaked mushrooms; cut remaining scallions in 2-inch sections.
5. Heat oil. Brown scallion sections lightly and discard. Add chicken shreds and stir-fry until they lose their pinkness .
6. Add stock, salt, shark's fin, mushrooms, and remaining sherry. Bring to a boil; then simmer, covered, 30 minutes.
7. Blend cornstarch and remaining cold stock; then stir in to thicken soup, and serve.