Prep: 30 mins
| Cook: 55 mins
| Extra time: 20 mins, cooling
1.
Preheat oven to 425 degrees F. Bring large pot of water to boil.
2.
Boil the potatoes until tender, about 15 minutes. Drain and mash until smooth. Mix in the yogurt and season well with salt and black pepper.
3.
Remove the skin and bones from the fish and cut into bite-size pieces. Place in the bottom of a 13x9-inch baking dish. Add the prawns.
4.
Place the leeks and the fish stock in a saucepan and cook for 1 to 2 minutes over high heat. Sprinkle the flour over and mix well. Cook for a further minute in order to "cook out" the flour.
5.
Add the mustard and beat well. Gradually add the skimmed milk, stirring continuously to prevent any lumps from forming. Bring to the boil, allowing the sauce to thicken. Pour over the fish.
6.
Allow to cool for 20 minutes, then cover the fish mixture with the mashed potato, either using a fork or piping using a piping bag with a large star-shaped nozzle.
7.
Top with cheese, then place in oven for 30 to 40 minutes until golden.
8.
Just before serving, sprinkle with the chopped parsley.