Step 1

Slice fish into six pieces; season all over with salt.

Step 2

Combine kombu (if using), sake, soy sauce, mirin, and ¾ cup water in a medium donabe or Dutch oven with lid. Bring to a boil, then reduce heat to low. Layer cabbage stems, then leaves in pot. Scatter mushrooms and ginger over, then place fish over mushrooms. Cover pot and steam until fish is opaque and just cooked through, 8–10 minutes.

Step 3

Drizzle with sesame oil and top with scallions. Serve in shallow bowls with broth spooned over and rice alongside.