Preheat the oven to 200ºC, fan 180ºC, gas mark 6.
Place the potatoes in a 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, mix the crème fraîche with the spring onions, parsley, lemon zest and cornflour.
Fold the fish pie mix into the crème fraîche mixture and season generously with salt and freshly ground black pepper.
Spoon this mix into a 1.2-litre ovenproof dish and cover with the mashed potato.
Sprinkle over the cheese and cook on a baking tray in the oven for 30-35 minutes, until bubbling and golden.