Method

1. Preheat oven to 200ºC (180ºC for fan assisted ovens) 400ºF, Gas Mark 6.
2. Place your fish in an oven proof dish, cover with milk and poach in the oven for approx. 18 minutes.
3. Remove fish from the milk (reserving the milk) and break into large pieces returning to the oven proof dish, with the prawns.
4. Melt the butter in a saucepan and gradually stir in the flour, allow to cook for 1 minute, then stir in remaining milk, bring to boil stirring all the while to thicken the sauce. Remove from heat and allow to cool. Meanwhile chop the watercress finely, stir into the cooled sauce and season to taste, then pour over the fish in the dish.
5. Unroll the pastry and cover your dish fully which may overlap depending on the size of your dish. Carefully trim the edges with a sharp knife, then press the edges of the pastry firmly down around rim. Make a small hole in top to allow steam to escape and brush pastry with beaten egg.
6. Bake for 20-25 minutes until pastry is golden brown.
7. Garnish with remaining chopped watercress and serve hot.