Put the codfish (or other white fish) in a pan with the onions, vinegar, ginger root, spices, wine and water. Bring it gently to the boil and simmer for 10 minutes. Add the scallops and prawns and cook for a further 3 minutes. Remove the fish; bone and skin the white fish and set it all aside.
Strain the cooking juices and set aside to cool for several hours by which time a lot of the sediment will have settled in the bottom of the bowl. Carefully pour off the juices, leaving the sediment, and then strain several times through a clean tea-cloth. You should have approximately 3 cups of liquid left.
Melt 3/4 oz of gelatin in a little of the liquid, cool it to room temperature, then mix it into the rest of the juices. Pour a thin layer (1/2 inch) of the juice into the bottom of a 5 cup soufflé dish or fish mold and put it in the fridge to set.
Flake the white fish into smallish flakes; remove the coral from the scallops and cut the white flesh into three of four pieces. Once the jelly is firm, arrange the most decorative of the fish in the bottom of the dish-- some scallop coral in the middle, prawns around the outsides, flakes of white fish in between or however you feel inspired.
Spoon a little more of the juice and return it to the fridge to set. Continue to layer the fish in the mould, setting each layer with a covering of juice until you have used up all the fish and juices. Leave the jelly to set for at least 4 hours in a fridge. Unmold and decorate with fresh herbs; serve as a starter.