Thoroughly rinse the fish in cold water and pat dry. Put the fillets in a 9-inch shallow heatproof bowl. Drizzle the soy sauce, salt and pepper over the salmon. Sprinkle with the scallions and ginger. Cut the lemon in half crosswise. Cut half a lemon into 4 slices and put on the fish. Juice the remaining lemon and drizzle over the fish.
Put a 1-inch-high steamer rack in a 14-inch flat-bottomed wok. Add water to a depth of 3/4 inch and bring to a boil over high heat. Carefully put the bowl on the rack, cover, and steam for 8 to 10 minutes. Test the fish for doneness by poking the thickest party with a chopstick or fork; the fish should flake. If not, steam 1 to 2 minutes or until the fish just flakes. Be sure to check the water level from time to time and replenish if necessary, with boiling water. Carefully remove the bowl from the wok. Drizzle the sesame oil over the fillets.
Recipe courtesy Grace Young and Alan Richardson, The Breath of a Wok: Unlocking the Secrets of Chinese Wok Cooking Through Recipes and Lore, Simon and Schuster, 2004