Double the ingredients in this easy fish pie if you need to feed four, or quadruple for eight. This comforting dinner is perfect for midweek, and adaptable for any situation
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.