1. Preheat your oven to 200c/Gas Mark 6
2. Put the potatoes in a large pan of cold, salted water and bring to the boil. Cook for 10 minutes or until soft, then drain and put back in the pan.
3. Using the 25g of butter and a splash of milk, make a smooth mash and leave to one side after seasoning with a little salt and black pepper.
4. In a large pan, fry the onion with the remaining butter and a touch of salt. Once soft, stir through the flour until a paste forms then gradually whisk in the milk bit by bit until you have a thick sauce.
5. Stir through the mustard and season to taste with salt and pepper. Add the fish pie mix and prawns (defrost first if frozen).
6. Put in a baking dish then top with the mash potato. Place in the oven and cook for 25 minutes or until browned.
7. To take it to another level, sprinkle on some breadcrumbs when you place it into the oven for a little crunch and finish with a sprinkle of parsley.