Cook the potatoes – Boil the potatoes in salted water until tender, then drain well. Mash with butter and milk until smooth and set aside.

Soften the leeks – Melt a little butter in a pan, add the sliced leeks, and cook gently until soft and sweet, not browned.

Make the sauce base – In a saucepan, melt the butter, stir in the flour, and cook for 1 minute to form a roux.

Add the milk gradually – Pour in the milk slowly, whisking constantly until thick and smooth, then stir in the cream.

Season lightly – Add salt and black pepper, keeping flavours gentle so the fish stays the focus.

Combine the filling – Add the fish pieces and softened leeks to the sauce, stirring carefully to avoid breaking the fish.

Assemble the pie – Spoon the fish mixture into a baking dish and spread the mashed potatoes evenly over the top.

Bake until golden – Bake in a preheated oven at 200°C (180°C fan) for 25–30 minutes until bubbling and lightly golden.

Rest briefly – Let the pie stand for a few minutes before serving so it settles nicely.