Preheat the oven to gas 7, 220°C, fan 200ºC.
Dry the fish with kitchen paper and using a sharp knife, make 3-4 deep cuts on both sides of the fish to the bone. Rub the salt and sesame oil into the cuts. Place a large piece of foil on a baking tray and place the pak choi on top.
Mix together the spring onions and ginger and place some into each cavity of the fish. Place on top of the pak choi, then scatter the remaining onions and ginger over the top. In a small bowl, mix together the rice wine and soy, then pour over the fish. Fold over the foil to make a parcel sealing the ends, then cook for 20 minutes or until the flesh is opaque.
Serve immediately with the juice and steamed rice.