Remove the discoloured outer leaves from the leeks and trim the tops and bases. You can keep the green bits for soup. Cut the leeks into 2.5cm lengths and wash really well.
Melt 15g butter in a heavy-bottomed saucepan and cook the leeks over a medium heat for about 5 minutes. They shouldn’t colour. Add a couple of tablespoons of the cider and cover the pan. Cook on a low heat for 7 minutes until the leeks are soft but still hold their shape.
Heat the milk in a small saucepan or frying pan (one that can hold the fish in a single layer) until it is gently simmering. Add the fish, cover with a lid and poach until the fish is cooked, about 4-8 minutes (depending on the thickness of the fillets).
Lift the fish out of the milk and break it into big chunks (remove the skin from the fish if it has any). Retain the milk.
Melt the rest of the butter in a saucepan and stir in the flour. Cook over a medium heat for about a minute to form a roux. Take the pan off the heat and add the reserved milk a little at a time, stirring well after each addition to keep the mixture smooth.
After all the milk has been added, stir in the rest of the cider. Put the saucepan back on the heat and bring to the boil, stirring all the time, to form a thick sauce.
Once it starts to boil, turn the heat down and simmer the sauce for 3 minutes to 'cook out’ the flour. Season, then stir in the mustard, the leeks and their cooking juices, and the fish. Heat through thoroughly, add the cream if you’re using it and taste for seasoning.
Pour the mixture into an ovenproof pie dish and leave to cool.
Preheat the oven to 190C/gas mark 5. For the crumble, put the flour, breadcrumbs and butter into a bowl and rub the mixture between your fingers until little lumps form. Add the rest of the ingredients. Spread over the fish and leek mixture and bake for 30 minutes, or until golden brown. Serve immediately.
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