To prepare sauce, combine first 6 ingredients, stirring with a whisk.
To prepare fish, lightly score each fish fillet by making 3 (1/4-inch-deep) crosswise cuts with a sharp knife.
Combine onions and 1 tablespoon ginger, tossing well.
Rub about 2 tablespoons onion mixture evenly into slits of each fillet.
Sprinkle fillets with salt and black pepper.
Combine leek, carrot, and bell pepper; arrange half of leek mixture in a 10-inch pie plate.
Pour half of sauce over leek mixture; arrange fillets in a single layer over leek mixture.
Top fillets with remaining leek mixture; drizzle with remaining sauce.
Open a small metal vegetable steamer; place steamer upside down in a large, deep wok.
Add water; bring to a simmer. Wearing oven mitts, carefully place pie plate on top of inverted steamer.
Cover and cook 12 minutes or until fish flakes easily when tested with a fork.
Wearing oven mitts, carefully remove pie plate from wok.
Garnish with coriander sprigs, if desired.