In a saucepan, combine soy sauce, sherry, water, ginger and sugar, and bring to a boil. Cover and keep warm over very low heat.
Lightly coat a steamer basket with nonstick cooking spray.
Set the basket in a large stockpot with water filled to just below the basket. Add salmon. Place over medium-high heat, cover, and cook for 6 to 8 minutes, or until cooked through. Do not overcook.
While salmon is cooking, in a skillet over medium-high heat, add sesame oil. Sauté the spinach until bright green and wilted. Divide spinach in two large soup bowls. If necessary, remove skin from salmon. Place fish on top of spinach. Pour soy broth over fish and sprinkle with scallions, sesame seeds, and additional ginger, if desired.
Toasting sesame seeds adds a nutty flavor. Cook them in a dry skillet until fragrant and golden, shaking frequently to prevent burning.