To make the White sauce: Gently heat milk in a saucepan until it simmers. Mix the arrowroot and the white wine together and pour slowly into the simmering milk whilst whisking until a thick consistency is reached. Note: Do not let the milk boil whilst making this.
Using 2 single serve oval pie tins, (divide the ingredients between the 2 dishes.) Spread the mashed potato on bottom and to the edges to create a well in the centre.
Divide and place the fish and spring onions into eash mash well and spread the mustard over, sprinkle with tasty cheese and half the poppy seeds.
Spread the white sauce over the top of both pie tins. Sprinkle over with parsley and cover with the puff pastry. Fold in the pastry leaving a small gap around the edges to reveal some mash.
Brush with egg wash and sprinkle with poppy seeds. Bake in a preheated oven at 230°C for 15-20 minutes.