Place hake in a pan with salt, pepper, milk, and bay leaf.
Cover and poach over medium-low heat for 25 minutes until opaque.
Flake the cooled fish into a bowl, discarding all bones.
Strain poaching liquid and add extra milk to reach 500ml.
Fold fish, mashed potatoes, one beaten egg, and parsley together.
Divide into four portions and shape into patties with damp hands.
Dust cakes in flour, dip in beaten eggs, and coat in breadcrumbs.
Fry fishcakes in oil for 3 minutes per side until golden.
Melt butter in a pan and whisk in flour for 3 minutes.
Gradually whisk in reserved milk and cook until the sauce thickens.
Stir in cheddar cheese until melted and season to taste.
Serve fishcakes with the hot cheese sauce spooned over them.