Bring the sake to the boil in a small saucepan and flame to burn off the alcohol which might toughen the flesh of the fish. Add the maple syrup and miso and whisk until smooth. Remove from the heat and allow to cool before using, or pouring into a jar and storing in the fridge.
To marinate the fish, brush a thick coating over the whole fillet and refrigerate for a few hours, or for up to three days. The longer the fish has marinaded, the firmer the texture and the deeper the flavour.
To cook your fish
Put the marinated fillet skin side down on the board and secure with a few lightly knocked in nails around the edge. Place the board at an angle next to your fire, close enough to cook the fish in the radiant heat, but not so close that the wood burns before the fish is cooked. Keep a close eye on the fish, and keep basting with more marinade as it cooks which should take fifteen to twenty minutes. The fish will be cooked when the flesh feels firm and cracks and yields slightly when pressed.