Fish pie with potato and celeriac mash

Boil or steam the celeriac and potato until soft; drain. Mash the celeriac and potato with butter and warmed milk until smooth. Season to taste with salt and pepper.

Meanwhile, heat 60g of the extra butter in a large saucepan, add the leeks; cook, covered, until soft. Add the flour, cook, stirring, for about 2 minutes. Gradually stir in the milk; bring to the boil, stirring constantly.

Add the fish to the pan; simmer, covered, for 3 minutes. Gently stir in lemon juice and chives without breaking up the fish.

Preheat the oven to hot, 220°C (200°C fan-forced). Spoon the fish mixture into an ovenproof dish (2.5-litre/10-cup capacity). Melt the remaining butter. Top fish mixture with potato and celeriac mash; brush top liberally with melted butter. Place dish on an oven tray.

Bake the pie in a hot oven for about 25 minutes or until golden brown.