Preheat the oven to 200°C, fan 180°C, gas 6. Put the potatoes in a large pan, cover with cold water and bring to the boil. Cook for 15 minutes until tender, then drain and mash with the butter and milk.
Meanwhile, put the crème fraîche in a large bowl and mix in the spring onions, parsley, lemon zest and cornflour. Stir in the fish pie mix and season with freshly ground black pepper.
Spoon the pie filling into a 1.2-litre ovenproof dish. Top with the mashed potato and sprinkle the Cheddar over the top.
Put on a baking tray and bake in the oven for 30-35 minutes until golden and bubbling.