The Nursery Fish Pie calls for smoked and fresh haddock. Are they both cooked in the milk? The recipe method doesn't distinguish and just says "simmer the fish".
Nigella's Nursery Fish Pie (from FEAST) is a British comfort dish of poached fish coated with a creamy sauce and topped with mashed potatoes. The pie uses a mixture of smoked and fresh haddock. The smoked haddock imparts some of its smoky flavour into the milk, which is then used to make the sauce.
Both the smoked and fresh haddock are poached in the milk. If you use a large frying pan you should be able to poach both the smoked and fresh haddock at the same time. But if you don't have room to cook them at the same time, poach the smoked haddock first, remove it from the pan and then poach the fresh haddock in the same milk. The milk will have been infused with the flavour of the smoked haddock and this will help to add a little extra flavour to the fresh haddock as it cooks.