Peel and chop the potatoes into smallish chunks then boil for 10-15 minutes or until tender

Drain the potatoes, then add the butter and milk, and most of the cheese. Mash until a smooth consistency is achieved

Pre-heat the oven to 200°C / 400°F / Gas Mark 6

Place the fish in a large frying pan with the milk. Stud each quarter of onion with a clove and place in the pan along with the bay leaves

Bring the milk just to the boil, then turn down and simmer for 8-10 minutes until the fish is cooked

Lift out the fish and remove any skin. Flake into a large baking dish (approx 2-2½ litre). Pour the milk through a sieve and leave to cool

Melt the butter in a clean pan then add the flour, and cook for a few minutes. Take off the heat and gradually add the cooled poaching milk, whisking between each addition to ensure no lumps remain

Return to the heat and bring to the boil. Cook for about 5 minutes, or until the sauce thickens enough to coat the back of a spoon

Throw in the chopped dill, nutmeg and season with salt and pepper. Pour over the fish and stir gently, so as not to break up the fish, just until the fish and sauce are fully combined

Pipe or spoon the potatoes over the fish and rough up the surface with a fork. Sprinkle with the remaining cheese, then bake for 40 minutes