Roll out the pastry and line a 20cm tart tin. Chill the pastry case for 30 mins, then bake blind in a preheated oven at Gas Mark 6 for 15 minutes. Remove the paper and beans and return to the oven for 5 minutes.
Put the fish, milk and bay leaf in a saute pan and bring to a gentle boil. Simmer for 3 minutes, remove from the heat, cover with a lid and leave until cold.
Strain the liquid into a jug, discard the bay leaf and set aside.
Flake the fish into large pieces and put them in the tart case with the spring onions and prawns.
Put the reserved milk and cream in a saucepan with the flour and butter. Stir constantly over a low heat until the sauce thickens and reaches boiling point.
Allow to simmer for 2-3 minutes, then remove from the heat and stir in the parsley. Season well with salt and pepper and pour over the fish.
Sprinkle the breadcrumbs over the top and bake at Gas Mark 6 for 20 minutes.