Preheat the oven to moderate 180°C (350F/Gas 4). Steam or boil the potatoes for 10-15 minutes, or until just tender (pierce with the point of a small sharp knife—if the potato comes away easily, it is ready). Drain, return to the pan and mash with the milk, egg and butter. Mix in half the cheese, set aside and keep warm.
Put the fish in a frying pan and cover with the milk. Bring to the boil, then reduce the heat and simmer for 2-3 minutes, or until the fish is cooked through. Drain, reserving the milk.
Melt the butter in a pan and cook the onion and garlic for 2 minutes. Add the flour and cook, stirring, for 1 minute, then remove from the heat. Gradually add the reserved milk, then return to the heat, stirring constantly, until the mixture boils and thickens. Reduce the heat and simmer for 2 minutes. Add the lemon juice, rind, dill, salt and black pepper.
Put the fish into a 1.5-litre ovenproof dish and pour in the sauce, mixing gently. Spread the potato over the top and sprinkle with the remaining cheese. Bake for 35 minutes, or until the top is browned.
