Set a steamer in a wok or pot with 2 in. water underneath. Bring water to a boil.
Meanwhile, boil sake and mirin until reduced by half. Mince 1 tbsp. lemongrass; cut the rest into thin 3-in. strips. Add minced lemongrass to sake mixture, reduce heat to medium, and add onion, garlic, juices, and zest. Cook 3 minutes; add chili sauce and cook until thickened, 10 minutes.
Put lemongrass strips and ginger in steamer. Top with fish; cook, covered, until just opaque in center, 8 minutes. Serve over rice with a drizzle of sauce and the cilantro.
*Find chili sauce in the Asian foods aisle. For sustainable fish choices, go to seafoodwatch.com