Trim the tough outer leaves from the bok choy and discard. Rinse the stalks and leaves and drain. Cut the stalks in half lengthwise. Cut the halves diagonally into 2-inch sections. In a bowl, toss the scallions and ginger with the bok choy sections. Arrange on a heatproof platter.
Preheat the oven to 450 degrees. Place the salmon steaks on top of the greens. Pour several inches of water into a roasting pan and heat until boiling. Carefully place the platter of salmon and vegetables above the water, on a rack or steamer tray. Cover the top of the pan tightly with aluminum foil. Steam 7 to 9 minutes, or until the fish is cooked. Note: If you have a bamboo steamer you may prefer to use that, instead of the roasting pan with water.
In a separate bowl, mix the soy sauce, Chinese black vinegar, sugar, and garlic. Serve the salmon from the heatproof platter or arrange the steamed vegetables and salmon on serving plates. Spoon some of the dressing on top and serve with steamed rice.
Recipe courtesy of Nina Simonds from A Spoonful of Ginger: Alfred A. Knopf, 1999