Preheat the oven to 180°C.
Place the Fish Fingers on a baking tray and bake until crispy and golden.
Meanwhile heat half the butter in a saucepan until foaming. Add the carrots and cook for around 5 minutes, then add the leeks and continue cooking until soft.
Add the rest of the butter and flour and cook out for a minute or so until smelling biscuity.
Pour in the milk gradually, beating it in with a wooden spoon, until you have a thick sauce. Bring to a simmer and cook for a few minutes then add most of the parmesan.