For the sauce, melt the butter in a large saucepan, add the flour and mustard powder and stir
until smooth. Cook over a very low heat for 2 minutes. Slowly beat in the milk, while heating,
until smooth. Simmer gently for 2 minutes then stir in the cheese until smooth. Remove from the
heat and cover the surface of the sauce with cling film. Set aside.
Meanwhile, boil the whole potatoes in plenty of salted water for 15 minutes. Drain well and set
aside until cool enough to handle.
Heat the oil in a clean saucepan. Add the onion and cook for 5 minutes until softened. Add the
leek, carrot, celery and mushrooms and cook for a further 10 minutes, or until the vegetables have
softened. Stir in the lemon rind and cook briefly. Add the vegetables to the sauce with the fish,
prawns, parsley and dill. Season and transfer to a greased 1.7 litre / 3 pint casserole dish.
Peel the cooled potatoes and grate them roughly. Mix with the melted butter. Cover the filling
with the potato and sprinkle with the cheese. Cover loosely with foil and bake in a preheated oven,
200ºC / 400ºF / Gas Mark 6, for 30 minutes. Remove the foil and bake for a further 30 minutes, or
until the topping is golden and the filling is bubbling. Garnish with dill sprigs and serve with