Poach the fish in the milk for 5-7 minutes until tender.
Remove the skin and any bones from the fish, then flake.
Fry the finely chopped onion in butter until transparent.
Combine the fish, mashed potato, onion, chopped parsley and capers, ensuring that the fish remains in flakes, season well.
Shape into cakes and dip in beaten egg, then into well season flour, then back into the egg and coat with breadcrumbs.
Grill gently for 5 minutes on each side until browned.