Slice fish into six pieces; season all over with salt.
While fresh shiitake mushrooms are definitely softer and cook quickly, the properly rehydrated dried mushrooms have a deeper flavor. Place the dried shiitake mushrooms in a heatproof bowl and add really hot or even boiling water to cover it by a few inches. Let the mushrooms sit for at least 20 minutes until the caps are tender.
Trim off and discard the tough stems, and the caps are ready to cook. Slowly pour the mushroom soaking liquid into a measuring cup and stop when you reach the gritty stuff at the bottom.
Soak dried goji berries in hot water for a few minutes until they've softened.
Combine kombu (if using), sake, soy sauce, mirin, and ¾ cup mushroom soaking liquid in a medium donabe (clay pot) or Dutch oven with lid. Bring to a boil, then reduce heat to low.
Layer the cabbage stems, then the leaves in the pot. Scatter goji berries, mushrooms, and ginger over them. Then place fish over the mushrooms.
Cover pot and steam until fish is opaque and just cooked through for 8–10 minutes.
Add the Japanese Udon to the broth and cook for 5 minutes.
Drizzle the sea-bass with sesame oil and top with scallions.
Serve in shallow bowls with broth spooned over and Udon alongside.