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 cilantro sprigs, if desired.
Wine note: The delicacy of the fish, the citrusy flavors, and the lightly exotic touch of ginger all demand a wine that is itself light, delicate, dry, and fruity: riesling. It's also a boon that riesling is high in acidity--always a great counterpoint to the oils in fish (that's why we squeeze lemon on fish). Of all rieslings, German wines are the most delicate and fruity. There are many great German rieslings available in wineshops; be sure to buy one labeled "Kabinett" or "Spätlese." A kabinett riesling is the lightest; spätlese is more full-bodied. Selbach-Oster, Dr. F. Weins-Prüm, and Dr. Loosen are all terrific producers. Prices start at $13. -Karen MacNeil