Cover dried mushrooms with boiling water and soak for 20 minutes. Strain and keep mushroom water. If using fresh mushrooms, remove stems and set aside.
Slice salmon filets lengthwise in half. Arrange on double think aluminum foil with edge turned up on top of the steamer.
If using dried mushroom remove from water, strain mushroom liquid through sieve lined with cheesecloth.
Combine scallions, ginger strips, vinegar, soy sauce, mirin, and 1 tbls of mushroom liquid. Add white pepper to taste. Spoon over salmon filets.
Remove stems and slice mushrooms into thin strips. Sprinkle mushrooms over salmon fillets.
Place steamer over boiling water and steam for 6 to 8 minutes. For wild salmon, you want at least 125F and no more than 145F.