Prepare the fish by removing all skin and bones and cutting into mouth-sized pieces. Put into a saucepan with the wine, water and stock cube. Put a lid on the pan and bring to the boil. Simmer for 2 minutes and turn off the heat.
Remove the fish with a slotted spoon and transfer to a bowl. Pour the liquid into a jug or bowl. Melt the butter in the saucepan and