The celeriac adds a little twist to the traditional potato topping for this fish pie. Yummy.
Boil the potato and celeriac in salted water until soft. Drain and mash with 25g of the butter. Add a little milk too if you like but watch the mixture does not become sloppy. Season to taste.
Meanwhile heat 60g of the butter in a pan, add the leeks and cook, covered, until soft. Add the flour and cook, stirring, for 2 minutes. Gradually add the milk and bring to the boil, stirring constantly.
Add the fish to the pan and simmer, covered, for 3 minutes. Gently stir in the lemon juice, herbs and seasoning. Try not to break up the fish too much.
Preheat the oven to 220C. Spoon the fish mixture into an ovenproof dish. Melt the remaining butter. Spoon the potato and celeriac mash over the fish and smooth with a palette knife. brush the top with the melted butter.
Bake for about 25 minutes until the top is golden brown.