Tip the potatoes into a pan, cover with hot water from the kettle and add a pinch of salt. Simmer for 15-20 mins until tender, then drain and leave to steam-dry.
Meanwhile, warm the butter in a heavy-based pan over a low heat. Add the flour and stir until you have a paste. Gradually whisk in 200ml milk until you have a thick white sauce. Simmer for 3-4 mins, stirring. Season, then stir in the mustard and half the cheese, and continue to stir until the cheese has melted. Fold in the fish, peas and kale, and cook for 8 mins more, or until the kale wilts, the fish is just cooked and the prawns are starting to turn pink.
Heat the grill to medium-high. Mash the potatoes with the spring onion and 1 tbsp milk. Fold in the remaining cheese and season well.
Tip the filling into a heatproof dish and spoon the cheesy mash on top, swirling it with a fork until the filling is completely covered. Sprinkle over the parmesan and grill for 10-15 mins until the pie is golden and bubbling. Serve with a green salad, if you like. Uncooked pie, well covered in the dish, will keep in the freezer for up to three months.