Heat the vegetable oil in a large pot over medium-high heat. Add the scallion whites and cook until slightly tender, 2 to 3 minutes. Stir in the ginger and cook about 1 minute. Add 7 cups water, the soy sauce, sugar, salt to taste, 3/4 teaspoon pepper and the mushrooms. Cover and bring the broth to a simmer. Mix the cornstarch with 1/4 cup cold water and gradually stir into the broth. Bring to a gentle boil, stirring; the broth will thicken slightly. Add the scallion greens, tomatoes and white fish to the broth, and simmer until the fish is opaque, about 3 minutes. Add the scallops, vinegar, and salt and pepper to taste (the scallops will cook instantly from the heat of the soup). Ladle into bowls.
Photograph by Antonis Achilleos
Courtesy of Food Network Magazine