In a bowl, whisk together the scallions, soy sauce, vinegar, gingerroot, vegetable oil, 1 tablespoon of the sesame oil, sugar, garlic paste, and red pepper flakes. Season with salt and pepper, to taste. In a shallow dish, arrange the halibut fillets in 1 layer, pour the soy sauce mixture over them, and let the fish marinate, covered and chilled, for 30 minutes.
Arrange a steamer or baking rack in a wide deep kettle and add water to the kettle to reach just below the steamer rack. Put a plate, such as a glass pie plate, at least 1-inch smaller in diameter than the steamer on the rack and place the fish on top. Bring the water to a boil. Pour the marinade over the fish.
Steam the fish, covered, over the boiling water until it just flakes, about 12 minutes, and with oven mitts, remove the steamer from the wok. Transfer the fillets carefully to a heated platter and drizzle them with the remaining 1 teaspoon sesame oil and the sauce remaining on the plate.