Arrange small flameproof dishes on a baking tray.
Mix breadcrumbs, cheese, and parsley for the topping and set aside.
Simmer milk, onion, bay leaf, lemon, and fish in a frying pan for 6 minutes.
Drain the fish over a jug and reserve the infused milk.
Melt butter, stir in flour, then gradually whisk in the reserved milk until thick.
Incorporate wine, cheese, mustard, and cream into the sauce and season it.
Flake the fish, discard aromatics, and fold fish into the sauce.
Divide the mixture into warmed dishes and sprinkle with the breadcrumb topping.
Grill for 5 minutes until the topping is browned and the sauce is bubbling.