1 Heat the oven to 180ºC (160ºC fan) 350ºF, gas 4.
2 Place the fish in a wide pan with the milk and bay leaf, bring to a boil and simmer gently for 5 minutes. Strain through a colander, reserving the milk, and set the fish aside. Discard the bay leaf.
3 Melt the butter in a saucepan and add the flour. Cook gently, stirring all the time, for 1 minute then gradually add the milk and bring to a boil.
4 Add the cream, season with salt and pepper and simmer gently for about 10 minutes then stir in the lemon juice.
5 Place the fish in an ovenproof dish and pour over most of the sauce, reserving about 250 ml.
6 Place the spinach on top, pour over the remaining sauce and scatter over the breadcrumbs and parmesan cheese.
7 Bake in the oven for about 15 minutes.