Place oil in a small pot set over medium heat. Add bell peppers, ginger and garlic and cook 2 minutes. Mix in teriyaki sauce, sake, sugar and cornstarch mixture. Bring to a simmer, and then cover and reserve sauce on low heat.
Line a steamer, large enough to hold the fish in a single layer, with a perforated piece of parchment paper. Set in the fish and cover. Set the steamer over simmering water. Steam 5 minutes, or until the fish is cooked through. Plate the fish, top with sauce, sprinkle with green onion, and serve.