Soy and Ginger Steamed Fish

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.