Place a steaming rack in a large wok. Fill wok with water to 1 inch below the rack. Place a heatproof plate on rack, making sure plate doesn't touch sides of wok.
Season fillets with salt and pepper. Coat lightly on both sides with sesame oil. Sprinkle flesh side with chopped cilantro.
Place one fillet on the plate, skin side down. Sprinkle with some lemon grass or peel; sprinkle remainder around plate. Place second fillet, skin side up, on top of first.
Scatter scallions, ginger, and mushrooms on top of and around fish. Drizzle with soy sauce and vinegar.
Bring water to a boil, turn down to a steady simmer, and cover wok. Steam until the fish flakes at thickest part, 10 to 15 minutes. Serve directly from plate.