Put oven rack in middle position and preheat oven to 400°F. Put baking dish in roasting pan.
Rinse fish and pat dry, then rub inside and out with salt. Transfer to baking dish and sprinkle with scallion strips (white and pale green) and ginger.
Stir together soy sauce and sugar until sugar is dissolved, then pour over fish. Add enough boiling-hot water to roasting pan to reach halfway up side of baking dish.
Oil a large sheet of heavy-duty foil, then tent foil (oiled side down) over fish and tightly seal around roasting pan. Carefully transfer roasting pan to oven and bake until fish is just cooked through, 30 to 35 minutes.
While fish bakes, cut enough scallion greens diagonally into very thin slices to measure 1/2 cup (reserve remainder for another use).
Just before serving, remove foil from fish and sprinkle with scallion greens.
Heat wok over high heat until a bead of water vaporizes instantly. Pour oil around side of wok, then tilt wok to swirl oil, coating side, and heat until smoking. Remove from heat and immediately pour oil over scallion greens and fish.
a large shallow baking dish (about 15 by 10 inches) to fit inside a 17- by 12- by 2 1/2-inch roasting pan; heavy-duty foil; a well-seasoned 14-inch flat-bottomed wok