Meanwhile, combine sake and next 3 ingredients in large skillet deep enough to hold steamer rack. Bring liquid to boil. Reduce heat; simmer 5 minutes. Arrange fish on rack; sprinkle with salt and pepper.
Place rack in skillet. Top fish with onions; drizzle with soy sauce and sesame oil. Cover skillet; steam fish until opaque in center, about 5 minutes.
Remove steamer rack.
Mix cilantro into juices in skillet. Spoon rice onto plates. Top with fish, juices from skillet, and sesame seeds.
Per serving: calories, 367; total fat, 5 g; saturated fat, 1 g; cholesterol, 62 mg; fiber, 1 g