1. Begin by making the champagne sauce. Melt the butter and combine with the mirin, champagne and salt in a small pan. Bring to the boil then add the flour to the pan, whisking continuously until a smooth sauce consistency is achieved. Set aside until required.
2. Preheat the oven to 180°C/gas mark 4.
3. Clean the cod and dry with kitchen towel, then brush over a thin layer of soy sauce. Arrange the spring onions on a baking tray and place the cod on top skin-side down, then bake in the oven for 15 minutes.
4. Remove the baked cod from the oven and brush with honey. Place under a hot grill until golden brown.