Sauté thinly sliced fennel in butter, season, cover and cook gently for 10 minutes, stirring from time to time.
Add a sliced leek (which you have soaked in cold water for 20 minutes to remove any soil from between the layers),
cover and cook slowly for another 5 minutes. Pour in a good glass of dry white wine and a couple of large dollops
(heaped tablespoons) of double cream (or crème fraîche) and continue simmering until the sauce reduces and starts
to thicken a little. Then poke chunks of cod loin and shelled king prawns into the sauce, cover again and simmer
for another 6-8 minutes, until the fish is just cooked and the prawns are nice and pink. Check seasoning, garnish
with chopped parsley and the chopped fronds from the fennel, and serve with new potatoes.