Scatter cabbage in shallow serving dish or pasta bowl large enough to fit in steamer. Add; top with shiitake, onions, ginger, salt and pepper. Cover and steam over high heat until fish flakes easily when tested, 16 to 18 minutes.
Pour off liquid from plate into saucepan; bring to boil over medium heat. Blend soy sauce, cornstarch and 1 tablespoon (15 mL) water; add to pan and return to boil.
Stir in sesame oil; pour over fish. Garnish with coriander.