Start by removing the skin from your ling fillets and cutting them into smaller pieces.
Place the fish with the sliced leeks into a large saucapan. Add the milk, bay leaves and peppercorns and bring to the boil. Lower the heat and leave to simmer, very gently, for about 5 minutes, until both the fish and leeks are almost cooked. Turn off the heat but leave the lid on.
Set the oven to 180°C.
Finely chop the dill and tip it into a mixing bowl. On top of the dill zest a whole lemon and pour in the breadcrumbs. Season with a good pinch of salt and pepper and mix everything well.
In a small pan melt the butter, then pour over the crumbs. Stir to coat everything thoroughly.
Remove the fish and leeks from the milk, discarding the peppercorns and bay leaves but reserving the milk.
In a casserole dish, melt the butter and stir in the flour to make a roux. Gradually ladle in the milk and whisk the sauce together.
Chop the tarragon and add to the sauce. Then, add the fish and leeks and season lightly.
Finally, scatter the buttered breadcrumb mixture over the top and bake for around 25 minutes until golden and crisp.