Preheat the oven to 200°C/ 180°C fan/gas mark 6. To make the pie filling, melt the butter in a saucepan over a medium heat. When foaming, add the onion. Reduce the heat, and sweat the onions in the butter for around 5 minutes until soft. Add the flour and curry powder and continue to cook over a low heat for a couple of minutes, then start to slowly add the milk, stirring well after each addition to ensure the sauce is lump free. When all of the milk has been added, continue to cook for a few minutes until the sauce thickens. Remove from the heat, cover and set aside.
Meanwhile, make the mash. Put the potatoes in a pan of cold water and bring to the boil. Simmer for 20 minutes or until soft. Drain and return to the pan. Add the milk and mash together until smooth. Season with freshly ground black pepper.
Pierce the spinach bag and cook in the microwave for 2-3 minutes until wilted. Mix together the sauce, spinach and all of the fish and place in a 20 x 25cm pie dish. Top with the mash, roughing up the surface with a fork. Bake in the oven for around 30-35 minutes, until golden. Serve with peas on the side.