Poach fish and onion in milk for 10 minutes. Drain, reserve milk. Remove skin and bone from fish, set aside fish. Melt butter in a pan, remove from heat and stir in flour. Cook, stirring for 1 minute.

Gradually stir in the milk off the heat. Cook, stirring, until thick. Fold in the fish, prawns, parsley, cheese, and season. Spoon into a 2 pint (1 1/2 litre) pie dish. Roll out the pastry and cut a strip to fit the rim.

Dampen the rim and top with remaining pastry. Seal edges. Re roll pastry trimmings, cut out 2 fish shapes.

Glaze with beaten egg and bake at Mark 7 (220°C) 425°F for 30 to 35 minutes until puffed and golden.