Remove the fish from the fridge and season with salt. Leave it to come to room temperature for 20 minutes.
Meanwhile, pre-heat the oven to 185ºC with the fan on.
Take a medium pot of water and bring to the boil. Add salt and blanche the onions for 1 minute. Remove and set aside for 2 minutes before adding to ice water to cool.
Once cooled, remove the outer layer of the onion and cut into quarters lengthwise.
Chop the ginger into small pieces, combine with the miso and fish stock and blitz in the blender until smooth. Season with vinegar and a touch of olive oil. Set aside.
Put the fish into a fish rack and cook for 15 minutes with the fan on full. Then, turn your oven to grill setting for 3 m to add colour and texture to the skin.
Then turn the oven off and allow the fish to rest inside for a further 5 minutes.
Sauté the onion in a large pan with olive oil. Heat the blitzed sauce in a pot.
Remove the fish from the oven and portion. Plate with the garnish and sauce on the side.