1 Melt the butter in a large saucepan set over a medium heat until hot. Whisk in the flour until a smooth roux forms.
2 Cook for 1 minute, and then whisk in the milk in a slow, steady stream until the sauce is smooth. Cook for 5 minutes over a reduced heat, until thickened. Season to taste with salt, pepper and nutmeg.
3 Add the cod and pasta, stir well, and pour everything into a slow cooker.
4 Cover with a lid and cook on a low setting for 4 hours.
5 Preheat the oven to 180°C(160° fan)|350F|gas 4.
6 Stir the spinach into the fish and pasta. Spoon into a baking dish and top with the breadcrumbs and Parmesan.
7 Bake for 40-45 minutes until golden on top. Leave to stand for 5 minutes before serving.