Put 25g butter, 25g plain flour and 4 finely sliced spring onions in another pan and heat gently until the butter has melted, stirring regularly. Cook for 1-2 mins.
Gradually whisk in 400ml milk using a balloon whisk if you have one. Bring to the boil, stirring to avoid any lumps and sticking at the bottom of the pan. Cook for 3-4 mins until thickened.
Take off the heat and stir in fish pie mix and king prawns, 1 tsp Dijon or English mustard, a small bunch of finely snipped chives,
Pour the sauce and fish mix into a large oven proof dish and top with the colcannon mash, brush the mash with the beaten egg and sprinkle over the grated cheddar,
Bake in the oven for 25-30 minutes or until the sauce is gently bubbling up the sides on 200C/fan 180C