Lightly salt the cod fillets and leave for a few minutes. Pat them dry with kitchen paper.
Place the cod in a shallow dish and pour over 2 tablespoons of the sake, turning the fish so it is coated on all sides. Leave to marinate briefly while you prepare the vegetables.
Arrange the sliced pak choi in a wide pan with a lid to form a base layer. Scatter the sliced shiitake mushrooms over the leaves.
Lay the cod fillets on top of the vegetables. Add the ginger juliennes and pour over the remaining 2 tablespoons of sake.
Place the lid on the pan and cook over a medium heat so the fish steams gently.
Halfway through cooking, scatter over the sliced spring onions and drizzle in the soy sauce. Replace the lid and continue steaming until the fish is just cooked through.
Serve immediately, spooning the fragrant cooking juices over the fish and vegetables. Finish with a drizzle of yuzu juice before eating.
