Melt butter in a saucepan

Add onion and cook until clear

Stir in curry powder

Cook for 30 seconds

Stir in flour and cook until frothy

Gradually add milk, stirring constantly until mixture boils and thickens

Remove from heat

Add fish, and pepper to taste

Stir then set aside until cool

Place a sheet of pastry on oven tray

Spread fish mixture over pastry, leaving a 2 cm edge all the way round

Dampen this edge lightly with water

Carefully fold second sheet of pastry in half

From centre fold make 1.5 cm wide cuts to within 2 cm of edge

Open pastry out and carefully lift over filling

Press edges firmly together

Lightly beat egg yolk and brush top surface of pastry with this mixture, making sure egg does not drip down sides as this will prevent pastry from rising

Cook at 220°C for 20 minutes or until golden and well risen